12 Lesson - Colour and Light Introduction A large part of the beauty and value of gemstones and precious metals revolves around the interaction between light and the object. This includes not only the hue and saturation of colour, but also how light is tra nsmitted, reflected, refracted, fluoresced, and dispersed. Light is electromagnetic radiation or energy, and can be described as behaving like both waves and particles (photon). Like all waves, light can be described by its wavelength, the distance from p eak to peak or trough to trough, and its frequency, the number of wave crests (or troughs) that pass through one point in one second. Light propagates in the direction of its wave front. All electromagnetic radiation (from radio waves to x rays) travels at a constant speed. So, when the frequency of light is decreased, its wavelength must increase this is an inverse relationship. Light energy increases with increasing frequency (or decreasing wavelength). Refer to the figure and table below for more information on the parts of a wave. Light also behaves like a particle when it travels as photon particles. More intense light would be composed of a greater number of photons with a higher frequency of incidence. For gemstones, interaction with light is best d escribed using the wave-like approach. For those students interested in the wave-particle duality of light, a good online starting point is Wikipedia. What is Colour? Colour is what our brain interprets from the incidence of light (electromagnetic radiation within the visible spectrum) on our eye. In other words, the colour of an object is our eye's interpretation of light in the visible range that has interacte d with the object we are looking at. The electromagnetic spectrum is continuous and represents radiation energy ranging from high intensity gamma rays (short wavelength, high frequency) to low intensity radio waves (long wavelength, low frequency). In the middle of this is the visible region which ranges from about 350 to 750 nanometers (nm). This range comprises the visible rainbow with which we are all familiar with: violet at the short end (~400 nm) and red at the long end (~700 nm). Remember the acronym ROY G. BIV? We use it to remember the sequence of colours of the visible spectrum: Red Orange - Yellow - Green - Blue - Indigo - Violet. Just outside of the visible region on the shorter wavelength end is the UV (ultraviolet) range and NIR range (near infrared) on the longer wavelength end. White does not appear as a colour in the spectrum because white light is a mixture of light with wavelengths across the visible range. Illumination Different sources of energy will emit electromagnetic radiation at different intensities across the electromagnetic spectrum. A simple example of this is a light emitting diode (LED) made to emit only one colour (monochromatic). In the case of a red LED, there would only be light emitted with a wavelength in the ~650 nm range and no light emit ted anywhere else along the spectrum. Light sources that are not intended to be monochromatic can have widely different spectral emittance curves depending on the composition of the source of energy we call the "light bulb". These spectral emittance curves describe the intensity of light at a particular wavelength and will often display data only across the visible spectrum (i.e., ~350 nm to ~750 nm). The figure below compares the spectral emittance curves (sometimes referred to as spectral distribution cur ves) of three different sources of light, emphasizing the difference in emitted colours across the visible range. Natural daylight (noon sunlight) is well balanced, incandescent lamps (heated tungsten filament) are skewed towards a warm red, and "Cool White" fluorescent lamps have distinct outputs in specific blue and green regions. This is why something you buy from a fluorescent-lit store (vegetables, clothes, jewellery) might look a little different when viewed outside in natural daylight or in your home under incandescent light. Businesses that depend on visual appearance to make their sales, such as in retail sales of jewellery, are very much aware of this and usually consult lighting experts to optimize conditions in their salesrooms. With respect to gemstones and jewellery, knowledge of how the intensity of light varies according to wavelength is very important when analyzing the resulting colour perceived by our eye. The "colour change" gemstone alexandrite (a variety of the mineral chrysoberyl) is a great example of this. Fine quality specimens will exhibit two distinct colours under specific lighting environments with distinct spectral emittance curves. Other gem varieties that exhibit colour change characteristics include garnet, corundum, and zultanite. Reflection and Refraction When light passing through one medium strikes another medium, part of that light is reflected (like a mirror) and the other part is refracted (like what you see through a fish tank). Reflection obeys a simple geometrical law where the angle of incidence is equal to the angle of reflection (<i = <r).
Refraction is different from reflection. When light passes from one medium into another, its speed changes causing light to "bend" or change in direction. The degree to which light is slowed and bent relate to the differences in the refractive indices between the two media as well as the angle at which the light path makes with the medium. The refractive index, n of a medium (such as a gemstone) is a measure of how much it will refract light of a specific wavelength passing from a vacuum into the medium in question. In other words, the refractive index measures how much the incident light is slowed compared to a vacuum environment (where the refractive index is, by definition, equa l to 1). The refractive index of a medium is also dependant on wavelength. Refractive indices of minerals and gemstones are used as diagnostic features in identification. Diamond's refractive index of 2.419 quickly sets it apart from regular glass with a refractive index of 1.5. When cracks or fissures are present in a rough gemstone, gem dealers will often attempt to fill them with an epoxy to strengthen the stone. This allows the material to be faceted into a larger gemstone. If the refractive index of t he epoxy is not matched with the host mineral's refractive index, the epoxy's different refractive index will cause the light traveling within the stone to refract (or bend). Because gemstones are normally homogenous, the refraction of light inside an 'epoxied' stone will be atypical and distracting, taking away from its value. If the epoxy's refractive index matches that of its host, no refraction will occur and the filler material will be optically undetectable. Consequently, a great deal of effort is invested in order to ensure that the refractive index of an epoxy matches that of the mineral that it is strengthening. Let's consider reflection one more time. When light travels from a medium with high refractive index to one with low refractive index, total internal reflection can occur if the angle of incidence is greater than the critical angle. The exact value of the critical angle is defined by the relative refractive indices of the two media. The result of this phenomenon is the light ray being completely reflected back into the higher index medium. This phenomenon has great significance for faceted gemstones. For example, a diamond (which has a high refractive index of 2.419) is faceted with specific angles and proportions to maximize the amount of lig ht that undergoes total internal reflection. Thus, much of the incident light entering the table of the gem is returned back through the table (and to your eye!). This plays a big role in determining the brilliance of a gemstone. Isotropic Minerals Recall from previous lessons that some minerals belong to the isometric crystal system and others to the monoclinic, triclinic, orthorhombic, tetragonal, and hexagonal crystal systems. Minerals that belong to the isometric system are also isotropic minerals because they have only one refractive index that is applicable in all 3-dimensional orientations. Examples of isotropic minerals with a single refractive index are diamond ( n=2.419) and spinel (n=1.725). Material from all crystal systems other than isometric show more than one refractive index and are termed anisotropic. Anisotropic Minerals and Double Refraction Anisotropic minerals are those that exhibit more than one refractive index. Materials that belong to the tetragonal and hexagonal crystal systems have two distinct refractive indices and those of the monoclinic, triclinic, and orthorhombic systems have three distinct refractive indices. The orientation of these refractive indices is related to the unique crystal structure of each mineral, and the absolute difference between the refractive indices is called birefringence, n:
A result of this anisotropy is that light entering an anisotropic medium will be split into two distinct light rays. In media with a high birefringence, the difference between the refractive indices is large and the difference in ligh ts paths is significant. Consequently, light transmitted through the medium appears "doubled" (see figure below). In media with a low birefringence, the difference between refractive indices is small and the difference in lights paths is minimal; consequently the resulting image looks more blurry than doubled. Dispersion We described earlier how light rays that pass from one medium to another undergo refraction and that the degree of refraction of light is dependant on its wavelength. Recall that "white" li ght is a mixture of light with wavelengths across the visible range. Thus, when white light refracts, its component lights are separated when the different wavelengths (colours) are bent at different angles. This is called dispersion. Longer wavelengths (e .g., red) are refracted the least and shorter wavelengths (e.g., violet) are refracted the most. This phenomenon of dispersion is what gives gemstones their fire. In gemology, dispersion is calculated as the difference in the refractive index for light of the shortest and the longest wavelengths. Because we are only dealing with light in the visible range, we use the refraction indices of violet and red,
430.8 and 686.7 nm, respectively. Gemstones with higher values of dispersion will show greater spreading, or dispersion, of colour. Most notable of the gemstones is diamond, which has a dispersion value of 0.044. Here, the refractive index for violet light is 2.451 and for red it is 2.407. Thus, the dispersion is 2.451 - 2.407 = 0.044. However, there are many other stones with higher dispersion values, for example, demantoid garnet = 0.057 and titanite = 0.051. How is Colour 'Generated' in Gemstones? Traditional ways of explaining colour often use the terms idiochromatic (or "self -coloured" from an essential constituent), allochromatic (or "other-coloured" from an impurity), and pseudochromatic (or "false-coloured" from physical optics). This straight-forward simplification of more complex interactions between light and the coloured medium works well for describing the main colours observed in gem materials. An element responsible for colouration of a mineral it is called a chromophore. Idiochromatic minerals have inherent colours that are derived from essential elemental constituents of their crystal structure. Peridot (Fe2SiO4) is an example of a transparent idiochromatic gem mineral where Fe is the chromophore. Turquoise (CuAl6(PO4)4(OH)84H2O) is an example of an opaque idiochromatic gem mineral where Cu is the chromophore. Allochromatic minerals do not have inherent colours, or at least not vivid colours, and require "impurities" to generate their colour. Emerald (Be3Al2Si6O18) is an example of an allochromatic gem mineral where Cr is the impurity that acts as the chromophore. (Recall from Lesson 7 why Cr is not listed in the mineral's chemical formula.) The tricky thing with allochromatic minerals, however, is that you can't just "shove" chromophore elements into a crystal, as we learned in the lesson on minerals. There needs to be an atomic site where a chromophore can substitute for a pre-existing element that is similar in ionic size and electric charge (not too big, not too small, but just right - the "Goldilocks Principle"). In the case of emerald, the base formula is Be3Al2Si6O18, shows it contains Be (normally +2 charge), Al (normally +3 charge), Si (normally +4 charge), and O (normally -2 charge). Chromium, normally a +3 charge, substitutes for the only +3 cation in the base formula, Al. Pseudochromatic minerals show colours and optical effects through dispersion and scattering of light. Colour and optical effects generated from scattering includes asterism, chatoyancy, iridescence, opalescence, and labradorescence. Colour generated from dispersion, as we learned earlier, is the result of light passing between media with varying refractive indices. Gemstones with higher values of dispersion will show greater spreading, or dispersion, of colour. Diamond is a well-known example of colour or "fire" generated through dispersion, although the minerals calcite, moissanite, sphalerite and zircon are all great examples as well. Asterism describes a prominent star shape that normally occurs as a six pointed star (although 8 and 12 are possible) and is due to crystallographically oriented mineral inclusions in the host mineral. Gemstones with this characteristic are best cut as "cabochons" (shaped and polished as opposed to faceted) to show off this optical effect and the most famous examples are in sapphires and rubies. Chatoyancy is the result of many fine fibre inclusions oriented in a parallel manner producing the well known "cat's eye" effect. This is similar to asterism, thus stones with chatoyancy are also usually cut as cabochons. Play of colour from internal scattering of light off of fine particles in a mineral is known as iridescence or opalescence, but is sometimes described as "schiller". It is commonly seen in the gems sunstone and opal. Labradorescence is similar to iridescence and is most commonly seen in labradorite, a species of the mineral feld spar. It is caused by diffraction of light interacting with very thin intergrown layers of calcium feldspar and sodium feldspar. The width of the thin layers defines the colour generated during diffraction. Pleochroism Some minerals will display different colours (or saturation of colours) depending on the crystallographic direction of the stone being viewed. This effect is called pleochroism and is caused by differential absorption of light according to orientation of the crystal. Tanzanite is an excellent example of a pleochroic mineral and displays three colours (often brown, purple, and blue) that align with the three different crystal axes. Iolite (the gem variety of cordierite, page 287 of your textbook) is another example. Its pleochroic colours are typically violet-blue and colourless. Transparency Transparency describes how light transmits through a medium. There are roughly five main groups, transparent, semi transparent, translucent, semi-translucent, and opaque. Transparent minerals are those where objects can be viewed through the medium (e.g., glass, diamond, beryl). Semi-transparent minerals are those where objects can be viewed through the medium, but object are heavily blurred (e.g., chalcedony, moonstone). Translucent minerals are those where objects cannot be viewed through the medium, although light will pass through the medium with lesser intensity (e.g., jade, opal, agate).
Semi-translucent minerals are those where objects cannot be viewed through the medium and light will only pass through the medium if it is thin (e.g., jade, turquoise). Opaque minerals are those where objects cannot be viewed through the medium, and no light will pass through (e.g., pyrite, malachite, galena). Basic Tools The following basic tools are commonly used by the weekend enthusiast for perusing pawn shops and rockhounding. Unaided eye Our most important and reliable tool is simply our eye, unaided and unhindered. It gives us immediate information about the colour of a stone, although that is not always diagnostic. A trained eye can estimate the dispersion of a stone, observe crystal habits, fractures, and cleavages, and identify characteristic inclusions and associated minerals. The eye also allows us to collect observations using other methods, and along with t he brain facilitates the combined contextual interpretation of all our observations. For a little boost, hand lenses and magnifying glasses are often used to look at the finer details. Tweezers A simple tool, but necessary for anyone looking at minerals, g ems or jewellery. A steady hand is tough to find, and even if you have one, the oils or sweat from your skin can alter the optical properties of the item at hand. Gemstone -specific tweezers also have a couple of adaptations to facilitate investigation incl uding textured tips (for grasping stones), small groves along the end of the tips (for a better grip of stones along their girdle), and locking mechanisms (so you don't drop a stone!). Other gem specific tweezer tools are stone holders. They resemble a cli cking pen with the point replaced with spring-loaded retractable prong claws and no stopping mechanism. They allow for a stone to be tightly held by more than two contact points, as with tweezers. Hammers and Chopsticks Rock hammers are less useful in a gemological setting (just try to bring one into a jewellery store!) but are essential for mineral or rock collecting. A simple carpenter's hammer will not suffice, it is important that an actual rock hammer is used because these tools are specifically hardened so as to not splinter when struck against a rock. Estwing is a common brand in the geological rock hammer world and they make a great 22 oz. hammer with a pick on the back end for prying open tough rocks. Personally, I prefer a Geotul, which is a hammer with a 30" long shaft capped by a 2.5-lbs hardened head that is backed by a flat ended pick. It breaks most rocks, and for those it can't, there is always the 10-lb sledge that stays in the back seat of the truck. For cleaning away dirt and debris from mi neral specimen in the field, chopsticks and paint brushes are very effective and usually soft enough that they will not damage any delicate crystals. Scratch Pad and Hardness Picks The hardness and streak of a gem is not easy to determine because it would require destructive techniques. For mineral specimens, however, there is usually enough material that some can be sacrificed to determine hardness and streak colour. Streak colour is independent of the mineral's apparent colour and can easily give away ce rtain minerals, such as hematite. We test streak colour by rubbing the mineral in question against a white ceramic plate (H=~6.5). If the mineral is softer than the plate, it will leave a streak of its powdered material on the white backing; the colour of the streak can be used to narrow down the possible mineral identities. Gem minerals tend to be harder than 6.5 and consequently streak plates are not always useful since a mineral harder than 6.5 will actually scratch the streak plate instead of vice-versa. In this case we can use known examples of other minerals to determine relative hardness, or we can use specially made "pencils" tipped with materials of known hardness. By finding out which known materials can and cannot scratch the unknown we can then determine the range of hardness that the unknown mineral must have. This is especially useful, for example, to determine whether a clear hexagonal mineral is quartz (H=7), beryl (H=8.5), or corundum (H=9). Gemological Tools Once certain gem identifications are required as part of a hobbyist's pastime or a gemologist's job, the following gemological tools become necessary for your box of tricks. Depending on the purpose of your tool box, not all of these tools are required. For example, a jewellery store is likely to have a portable diamond tester, whereas a gemological laboratory may pass on the portable diamond tester in lieu of more sophisticated equipment. Loupe A loupe (pronounced loop) is also known as a hand lens, or a handheld magnifier, and is used by every jeweller, gemologist, mineralogist, and geoscientist. The standard magnification power used is 10X, and this is what gemologists and mineralogists will use as a base tool for investigating minerals, grading stones, and examining jewellery. A 10X power loupe will create an image 10 times larger than real life. Most loupes are constructed as "triplets", meaning they
have three lenses mounted together that minimize distortion of the transmitted image. After your "eye", the loupe is the next most important tool for collecting visual information. Chelsea Filter The Chelsea Filter is also sometimes called an Emerald Filter since it is quite effective at discriminating emeralds from other green stones. It is simply a colour filter that only allows red and g reen colours to pass through the filter. It effectively filters out any blue hues of light passing through a stone and gives clues to the true nature of an unknown sample. Each main gemstone variety will show a specific colour or range of colours through the filter, thus adding another piece of information to the list when identifying an unknown. Dichroscope The dichroscope is a useful tool for determining what optic class a mineral or gem belongs to. It capitalizes on optical effects generated from gems with two or three indices of refraction when light is transmitted through the stone. It's essentially a tube in which two dichroic filters are set next to one another, but oriented 90 degrees from one another; these are usually made of calcite. The resulting effect is that stones with more than one refractive index (i.e., any material that is not isometric) will show two different hues through the two different filters (seen as little rectangles through the scope). Stones with only one refractive index (i.e., any mineral in the isometric crystal system, e.g., diamond or spinel) will only show one colour. This quickly differentiates these two classifications of minerals. Furthermore, the specific colours and tones seen through the dichroscope of dichroic (e.g., sapphire) and trichroic (e.g., tanzanite) minerals can also be diagnostic to a trained observer. With a hand lens, Chelsea filter, and dichroscope, nearly 90% of all gemstone varieties can be identified. UV Lamp These lamps emit light in the ultraviolet (UV) portion of the electromagnetic spectrum. They generally come in two types; short wave and longwave. Short wave UV lamps emit peak intensity around ~260 nm while longwave lamps emit peak intensity around ~365 nm. All conventional UV lamps are mounted with fluorescent tubes and many come with two tubes: one that emits longwave and one that emits short wave radiation. Long wave UV light emitting diodes (LEDs) are also becoming more common in the marketplace. UV lamps are used to observe UV fluorescence (under short wave and longwave) in gemstones and minerals, a diagnostic feature of many minerals. Certain minerals under UV radiation (which has a more energy that visible light) re emit the radiation at a lower energy level. If the energy level of the emitted light is in the visible realm, then our eyes will be able to detect it. This re-emitted light is called UV fluorescence. The word "fluorescence" comes from the mineral fluorite, which displays this behaviour under UV radiation. Refractometer Refractometers are used to determine the refractive index of a faceted stone through refraction and reflection of light. They are not usually "pocket-sized" tools, but come in compact portable versions as well as desktop versions. These use the same concepts of light as those by dichroscopes, i.e., the degree to which specific wavelengths of light will bend and slow down depends on the refractive index of the medium. However, the refractometer differs from dichroscopes in that it does not use transmitted light. This allows the user to determine the refractive index of translucent to opaque materials like jade, hematite, or turquoise. The information derived from a refractometer reading is objective and quantitative and can be quickly compared to tables. Consequently, this tool is very useful for difficult or unusual stones and, in combination with the loupe and Chelsea filter, allows the identification of almost all common gemstones. Diamond Testers Diamond testers, as the name suggests, are used solely for determining whether a stone is indeed a diamond or another material. Traditional diamond testers used diamond's superior thermal conductivity to differentiate it from any other stone. Over the years, diamond imitations have become more sophisticated with some having thermal conductivities that come very close to that of diamond. Consequently, newer tools also test for electrical conductivity. With these two pieces of information, diamond can be distinguished from most non-diamonds. However, synthetic diamonds or treated diamonds will also test positive, as they are the same compound. More specific tests are needed to distinguish these from natural diamonds, and it is best left to gemological laboratories that are set up with all the tools and toys! Microscope Microscopes are tools for magnification that are stationary and usually occupy desk-space. They are calibrated instruments with higher power magnification than a hand lens. They will usually have variable magnification lenses (usually from 10X to 100X) and include a focusing knob to allow investigation of different parts within a gem or mineral. Microscopes are typically binocular (have two eye pieces) and gemological microscopes will often have a variety of lighting types, including diffuse lighting and spot lighti ng, as well as a variety of lighting sources e.g., incandescent and full spectrum.
Compared to the human eye, microscopes are much more capable at finding flaws in stones because of its 100X magnification and observations under ideal lighting. Unfortunately, findings from microscopes are not always representative of how a stone should be graded. We learned earlier that as a rule, the value of a stone is generally based on its appearance to the unaided eye. This is why only a 10X loupe is usually used in grading gemstones. The microscope is the vehicle into the world of inclusions - the tiny gases, liquids, and solids that exist in every gemstone. Inclusions can take on a variety of shapes and sizes and can create truly beautiful patterns. A wonderful place to start investigating the natural art of gemstone inclusions is John Koivula's website: microWorld of Gems. Spectroscope - Pocket and Benchtop Models The spectroscope is a specialized gemological tool that is used primarily to differentiate specific stones from one another when results from other tests are not conclusive. The concept behind a spectroscope is based on absorption of light transmitted through the gemstone. White light passing through a stone will have some of its spectrum absorbed. As this transmitted light passes into the spectroscope tube it is separated out into its spectrum of colours by a prism or diffraction grating. Where a specific light has been absorbed by the material, dark spots will appear on the spectrum. The specific bands of light that are absorbed are characteristic for specific gemstones. Compendiums of absorption spectra are compiled in reference books for gemologists. These tools come in both pocket size and benchtop models. Pocket-sized spectroscopes are usually not considered quantitative but to a gemologist with a good understanding of the anticipated spectra of gemstones, certain species can be ruled out quickly if characteristic absorption lines are not present. Immersion Cells and RI Liquids Immersion cells or vessels, are designed for determining the refractive index of a gemstone. They can also be used to inspect a gemstone for diffusion treatments, and can quickly show if the stone at hand is a doublet or triplet (a composite stone). When a stone is immersed in a liquid of the same refractive index, any light that strikes the gemstone will not refract and passes directly through. This allows for any colour zoning in the stone to not be refracted and spread across the table facet (as you would see in natural light). The resulting image will clearly show whether or not the stone has an even colour saturation, natural colour zoning, or artificial colouration. In the case of a stone with an unknown identity, the gemologist can check its refractive index by using a set of fluids with known refractive indices. The gemstone in question is immersed in a series of liquids with increasing refractive indices until no refraction occurs. When this happens, the refractive index of the liquid is equal to that of the unknown gemstone. This establishes one more piece of information to help identify the unknown stone. Refractive index fluid sets often have increments of 0.005 and range from ~1.4 to ~2.0 (remember that diamond has a very high RI of ~2.418). More sophisticated systems allow the gemologist to control the refractive index of the fluid instead of placing the stone in subsequent drops of different fluids. Polariscope Polariscopes are essentially sophisticated benchtop dichroscopes in which a more controlled environment is created. This is particularly useful for stones with two or three refractive indices that are very similar (e.g., the quartz family of gems), and therefore will not show pronounced changes in a dichroscope. Polariscopes are often used in conjunction with immersion cells. Mineralogical Tools Polarizing Microscope The polarizing microscope is similar to a normal microscope but has a number of distinct differences. First, it is designed primarily to view rock samples than have been sliced to 30 microns in thickness, which are known as thin sections of rock. Next, it has a series of special filters that allows the user to polarize and change the light passing through the minerals in the thin section while observing how that light interacts with the individual minerals. Last, it has a rotating stage, variable focus, and high magnification (up to ~400X). These microscopes tend to be very expensive and are most often found in university laboratories. They are sometimes called petrographic microscopes. When more detailed chemical information is required about a gemstone it can be studied using very sophisticated mineralogical tools that utilize X-Rays and electron beams to probe the samples. Electron microprobes can determine precise chemical formulae of mineral specimen by interacting with individual atoms within the specimen. X -Ray diffractometers (XRD) can determine precise crystallographic structures of specimens by interacting with the crystalli ne structure of a specimen. Both of these techniques are highly advanced and quite exciting to perform! They probe the innermost portions of crystals and give insight to the existence of specific atoms and their interaction with surrounding atoms. LESSON 20:
Pegmatites supply the world with the best tourmaline, topaz, and beryl along with a large selection of other rare stones some so rare that their faceted varieties are only cherished by the few collectors who can get their hands on them. In addition to the wonderful gems that pegmatites produce, these rocks are also important hosts for rare metal deposits, including lithium (Li), tantalum (Ta), niobium (Nb), and tin (Sn). Pegmatites are known for growing the largest crystals and are the environment for many "largest crystal" records for specific minerals. In fact, the term pegmatite is used as a descriptor for igneous rocks with large crystal sizes. Pegmatites and their Mineralogy First, what is a pegmatite? Prof. "Skip" Simmons, a "pegmatologist" from the University of New Orleans, defines these wonderful rocks as intrusive igneous rocks that are texturally very coarse to gigantic in size. Your textbook describes them similarly and goes into a bit more detail. In the next lesson, we'll dive deeper into the geology of pegmatites but for now we'll start with the minerals that comprise pegmatites. The mineralogy of pegmatites is directly tied to their geochemistry and most pegmatites can be character ized by a base composition similar to granite but with significant enrichment in rare elements. The enrichment in rare elements typically facilitates the growth of rare minerals (one of the prerequisites for a valuable gemstone) that require these particular elements in their crystal structure. An example of one of these rare minerals that you're already familiar with is beryl. This mineral's chemical formula is Be3Al2Si6O18. In regular granite, sourcing the O, Si, and Al would not be a problem. Sourcing the Be is a bit more difficult. However, because Be is a "fairly common" element in pegmatites, beryl is a "fairly common" mineral in these unusual and exciting rocks. Other rare elements that are commonly enriched in pegmatites include: Li, Cs, Ta, Nb, Y, F, Rb, Sn, Ga, and B. What a list! The following table gives oxide composition of typical granite, common pegmatite, and gem bearing pegmatite. Note the increase in Li, P, F, B, Be, Rb, and Cs in pegmatites relative to granite.
Oxide SiO2 Al2O3 FeO + Fe2O3 TiO2 MnO H2O MgO CaO Na2O K2O Li2O P2O5 F B 2 O3 BeO Rb2O + Cs2O Total
Granite 72.34 14.34 1.81 0.26 0.02 0.36 0.37 1.52 3.37 5.47 99.86
Weight Percent Common Pegmatite 74.2 15.0 0.6 0.6 0.3 4.6 4.2 0.3 0.1 trace 99.9
Gem Pegmatite 70.22 17.2 1.76 0.28 0.39 trace 1.36 4.45 2.85 1.49 0.7 0.11 0.18 trace trace 100.36
The enrichment of these unusual elements leads to the formation of unusual minerals. Unless one is a mineralogist , many of the minerals that pegmatites host are completely unfamiliar, yet, because of their rarity, they often make it into the gemstone world. The rarest of the unusual pegmatite minerals normally go directly to rare mineral collectors. The following table lists many of the gemstones found in granitic pegmatites, sorted by abundance. The list contents were summarized by Prof. Simmons. Remember that a gem name might differ from the mineral name, e.g. aquamarine and kunzite. Table of Gemstones Found in Granitic Pegmatites, Sorted by Rarity (modified from Simmons, 2007)
Gemstone albite amazonite aquamarine elbaite fluorapatite goshenite indicolite lepidolite oligoclase quartz sanidine-orthoclase spessartine spodumene topaz triplite zircon achroite amblygonite chrysoberyl danburite heliodor kunzite lazulite liddicoatite montebrasite morganite petalite phenakite pollucite rubellite triphyllite verdelite beryllonite brazilianite euclase hiddenite londonite microlite pezzottaite rhodizite
Colour c g g-b c, p, g, b b-p-pur-g c b pur-p c c, p, sm, pur y, c o c-g b-c-p r, br, p c, p, br, g c pl y, c g-y cy y p, pur b p-r c, pl p-y p-o c c, p, y c p-r b-g g c, pl y y-g b-g g-y y-c y, r-br r y-c
Mineral / Group plagioclase group alkali feldspar group beryl tourmaline group apatite group beryl elbaite-schorl tourmaline lepidolite plagioclase group quartz alkali feldspar group garnet group spodumene topaz triplite zircon tourmaline group amblygonite chrysoberyl danburite beryl spodumene lazulite tourmaline group montebrasite beryl petalite phenakite pollucite elbaite, tourmaline group triphyllite elbaite, tourmaline group beryllonite brazilianite euclase spodumene londonite microlite beryl group rhodizite
Abundance C C C C C C C C C C C C C C C C R R R R R R R R R R R R R R R R VR VR VR VR VR VR VR VR
rossmanite simpsonite stibiotantalite
p-r, c, g y, o y,
tourmaline group simpsonite stibiotantalite
VR VR VR
Colours: c=colourless; g=green; y=yellow; b=blue; p=pink; r=red; o=orange; br=brown; pur=purple; sm=smoky; pl=pale. Abundance in pegmatites: C=common; R=rare; VR=very rare. As you can see, there are many gem minerals found in pegmatites, some much more common than others, and some more commonly known than others. In this Lesson, we'll learn about one of the most common pegmatite gems, tourmaline. The word tourmaline has its roots in the Sinhalese word turamali which roughly translates to "stone with many colours". What is Tourmaline and What are its Basic Qualities? Tourmaline is a complex borosilicate mineral group with hexagonal symmetry. It typically occurs in long slender crystals with a pseudo-hexagonal outline and euhedral crystals are common. Vertical striations down the crystal face are very common and can sometimes be used as a diagnostic feature. It has two poor cleavages, so when the stone breaks the surface is quite uneven. It is fairly dense (SG ~ 3.2) but not to the point that it concentrates readily in placers. A hardne ss of 7 makes the minerals of this group useable in jewellery. Tourmaline can be strongly coloured and hues include the entire spectrum of the rainbow, but opaque black is by far the most common. Tourmaline's range of intense colours, size of crystals, and often euhedral shape make this mineral group a collector's favorite. Most mineralogists have a confessed affection for tourmaline and the Mineralogical Society of America even uses the outline of a "watermelon tourmaline" in their logo. What is its Chemistry and Crystal Structure? The crystal chemistry of tourmaline is complicated; some refer to it as a "garbage bag" mineral because so many different elements can enter into the structure. The base tourmaline formula and the formula for schorl (the most common variety) are: base: XY3Z6(BO3)3Si6O18(OH)4 schorl: NaFe3Al6(BO3)3Si6O18(OH)4 In the base formula, the letters X, Y, and Z represent crystallographic sites with variable composition. Schorl and the other 13 accepted varieties of tourmaline result from different combinations of constituents in these three sites. For schorl, the X site is filled with Na, the Y with Fe, and the Z with Al. The Si6O18 grouping represents vertically stacked but isolated rings comprising 6 Si tetrahedron linked together on their corners. The BO3 grouping represents the essential boron (B) that is linked with three oxygen and which is also stacked vertically along the c axis. The three cations that sit in the Y sites cluster together, perched on top of the Si6O18 rings. The Z site, which is typically occupied by Al, forms an inter-penetrating linked network that separates each column of Si rings from each other. The X and OH groups in the structure occupy the space between repeating units of Si r ings and Y site clusters. The other main varieties of tourmaline are classified according to the element occupying the X site: alkali tourmaline (X=Na), calcic tourmaline (X=Ca), and vacancy tourmaline (no X-site cation). Most gem varieties belong to the alkali tourmaline group and arise from different transition metals in the Y site. Elbaite is the most common gem variety and it has both Li and Al in the Y site. Colourless elbaite is sometimes called by its historical name, achroite. What Colours can Tourmaline Have? How are These Colours Generated? What Gem Varieties Result? Similar to beryl, the colour in tourmaline is most commonly caused by transition elements substituting into the crystal structure for Al. In elbaite, the most common gem quality variety of tourmaline, the common substitutions that produce different gem varieties occur in the Y site for Al, but sometimes occur in the Z site. Prior to accessible chemical analyses, varieties were distinguished by their colour. As a result, many elbaite mineral specimens were erroneously classified. Today, mineral designations are strictly defined by their chemistry, although some of the historical names live on. The most commonly known gem varieties and their characteristic colour are listed in the table be low.
Variety dravite indicolite rubellite verdelite Paraba
Dominant Species elbaite elbaite elbaite, liddicoatite elbaite elbaite elbaite
Colour red blue deep pink to red green yellow electric blue
Likely Cause Fe+3 Fe+2 and Ti+4 Mn+2 and Mn+3 Fe+2, Cr+3, or V+3 Mn+2 and Ti+4 Cu+2
Watermelon tourmaline is a bi-colour variety of this mineral where a bright pink core (from is Mn) surrounded by a grass green rim (usually from Fe). This colour gradient is the result of changing geochemical growth cond itions where originally the system was Fe-deficient, leading to the Mn-dominated pink colouration. As the system evolved, Fe became increasingly available to the growing tourmaline thus changing the way light interacts with the Fe -rich portions. Sometimes the rim can also be coloured blue depending on the Ti content of the system. What Does it Look Like Rough? As noted previously, most tourmaline crystals have an elongated crystal habit and often form in a nice euhedral shape. Crystals will often have striations (small grooves) down the length of the crystal. During the last stages of pegmatite growth the environment can become quite acidic and corrosion of crystals can occur. Single crystals are common, as are "fans" of tourmaline consisting of many crystals growing simultaneously. Single crystals produce the best gem specimens. Tourmaline Recognition, Value, Treatments, and Synthetics How is Tourmaline Recognized and Distinguished from Other Materials? Rough tourmaline is quickly recognized by its prismatic habit, striations along the crystal's length, pseudo-hexagonal outline (see images in your textbook), and association with pegmatites and other pegmatite minerals. Beryl also has vertical striations and is also hexagonal, but it is harder, shows true hexagonal outlines, and less commonly forms fan shaped crystals or clusters. Quartz is also a hard hexagonal crystal but it shows prominent striations across the long axis. In identifying tourmaline fragments, look for strong dichroism, where the colour (and its saturation) observed down the crystal's long axis is much different than across the long axis. In tourmaline, the darker of the two colours tends to be oriented along the length of the crystal (along the c axis) while the lighter colour is oriented across the crystal. Of course, it will not be possible to tell which way the crystal is oriented when observing a water worn pebble. In this case, rolling the stone around while using a dichroscope usually makes the colours show. In cut stones, diagnostic properties include strong dichroism, refractive indices of ~1.61 to 1.66 and a SG of around 3. How is Tourmaline Valued? Tourmaline is considered a semiprecious coloured gemstone and fetches less than emeralds, sapphires, and rubies. Its variable saturation and colour make this gem hard to standardize prices for, but fine specimen s of unusual colour can rival the prices of the "Big 3" (emerald, ruby, sapphire). Unless of unusual colour, tourmaline should typically be quite clean of inclusions. Chrome tourmaline is usually valued at up to $400 USD per carat for a 1 carat stone, with stones reaching sizes of about 10 carats. "Normal" rubellite is on par with chrome tourmaline with similar restrictions to sizes and associated prices. Fine rubellite with deep red-purple colouration or vibrancy can fetch up to ~$1000 USD per carat for stones in the 2 -10 carat range. Blue indicolite tourmaline is commonly valued in the same range as rubellite. Bi -colour and yellow tourmaline is much more common and is priced normally in the $100 USD per carat range. Paraba tourmaline from Mozambique is about $1000 USD / carat for stones up to ~2 carats; above that size, prices jumps dramatically. Stones up to almost 100 carats have been produced but are exceedingly rare, achieving prices in the $4000 USD / carat range. The source of the original Paraba tourmaline is depleted, so stones verified from that location will demand a premium. These stones are the finest Cu-coloured type, and can reach values in excess of ~$15,000 USD per carat. What are Common Treatments for Tourmaline? Tourmaline rough is often heated to bring dark stones into lighter hues and to saturate lighter stones. Stabilization with epoxy is sometimes performed but much less common than with emerald. Irradiation of cut stones is uncommonly observed with fancy pink tourmaline. Can It be Produced Synthetically? What are its Imitations? Tourmaline can be synthesized in the laboratory but this is not normally done because of the abundant supply of natural tourmaline. Paraba-type, indicolite, and rubellite tourmaline are the most commonly imitated varieties since they command the highest prices. Spinel, bottle glass, and amethyst are the most common material passed for these varieties of tourmaline. Lesson 21: Topaz has been known throughout antiquity, however, it is thought that this word was applied to a variety of gemstones such as peridot, aquamarine, or citrine. Its name likely comes from the island of Zabargad, formerly called Topazos. Interestingly, Zabargad is a classic source for gem quality peridot not topaz. What is Topaz and What are its Basic Qualities?
True topaz is an aluminosilicate mineral containing fluorine (F); often, appreciable hydroxyl groups (OH)- replace F. It is part of the orthorhombic crystal system and usually forms prismatic crystals with an eight sided cross-section (similar in shape to a lozenge) that are terminated in a wedge-like fashion. Striations are common along the length of the crystal. A perfect basal cleavage makes this mineral difficult to work with in jewellery, however, it has a good hardness of 8, placing it above quartz and tourmaline but below beryl and corundum on the Mohs scale. It is fairly dense with a SG of ~3.5. Topaz crystals can reach considerable sizes and single crystals up to 10 m long and 3 m across have been found, weighing up to ~350 tonnes! Of course, these crystals would not be of gem quality. Gemmy ones hav e been found up to several hundred kilograms. These make for particularly large cut stones, often in the thousands of carats! The largest cut topaz is from Ouro Preto, Brazil, weighs 22,892.5 carats and is hosted in the Smithsonian Institution National Mus eum of Natural History. Although from Brazil, it boasts the name "American Golden Topaz". What is its Chemistry and Crystal Structure? Topaz's chemical formula is Al2SiO4(F, OH)2. The Si in the structure is tetrahedrally coordinated (has four oxygen anions around it) and the Al is octahedrally coordinated (has six anions around i t). What is unusual is that the anions of the Al octahedron is a mix of four oxygen and two fluorine atoms. Limited chemical variation is seen in topaz but some Cr has been noted in pink samples. What Colours can Topaz Have? How are These Colours Generated? What Gem Varieties Result? Topaz comes in a more limited range of colours than tourmaline, but still shows quite a variation and is colourless when pure. After colourless, lightly coloured brown, blue, and yellow are the most common colours; pink, red, and lavender round out the mix. "True" Imperial topaz has a vivid reddish-orange colour, however, this variety name has been commonly misapplied to duller cognac coloured topaz. Colours are mostly generated from colour centers in the crystal, where single "free" electrons sit in holes generated by site vacancies normally occupied by F. These colour centers can be natural and form during crystal growth or generated from irradiation either naturally from radioactive minerals or treated in the laboratory. Imperial topaz is the most valued of the varieties and were originally sourced from the Ural Mountains of Russia. Today, most of the production of Imperial topaz, or near-Imperial topaz, comes from the Ouro Preto mine in Brazil. Fine pink topaz from northern Pakistan is also well-known. What Does it Look Like Rough? Topaz commonly occurs as well-defined euhedral crystals with sharp edges. It is stable in acidic geological environments, so it does not suffer the same resorption effects from a corrosive fluid as tourmaline and beryl do. The lozenge shape is very common with flat tops, however, pointed terminations are not uncommon. Topaz Recognition, Value, Treatments, and Synthetics How is Topaz Recognized and Distinguished from Other Materials? Rough topaz is quickly identified by its crystal shape, mineral and rock associations, and hardness. Tumbled (a lluvial) topaz is identified best by its hardness, basal cleavage, and high density. Topaz can be easily confused with the multitude of minerals that have inherited its name as a modifier. Examples are "topaz quartz" (actually citrine) and "smoky topaz" (smoky quartz); topazolite is actually yellow garnet! Topaz with brown to reddish-orange hues can be confused with zircon, and light blue topaz is often mistaken for aquamarine and apatite. Pink topaz is easily confused with tourmaline, kunzite, and spinel. How is Topaz Valued? Because topaz is found in relatively large crystal sizes and responds well to irradiation treatment, prices per carat tend to be low for the more common varieties. Blue, colourless, and brown topaz for example, are usually valued around $10 25 USD / carat. Especially large samples will naturally, command a higher per carat price. Rich orange-red Imperial topaz is much less common and untreated stones of this variety normally range into the ~$1000 USD / carat for larger (~10 carats) stones. Pink to red topaz, which is even more uncommon, retails for up to ~$3500 USD / carat and rarely achieves sizes beyond 5-6 carats. What are Common Treatments for Topaz? Irradiation, heating, and coating are the most common treatments applied to topaz. Irradiation techniques will generally produce blue topaz from colourless material and intensify lightly coloured blue, yellow, and orange topaz. Variations in the type of radioactive source, and therefore energy level, for the irradiation process results in a range of colour saturation. Topaz can be heat treated to generate pink colouration in certain samples. Topaz is commonly coated by a thin film to produce a variety of optical effects. The composition and thickness of the film will define the change in optical properties, such as uneven modification of colours to produce a play of colours (e.g., Mystic Topaz). Some coated topaz is susceptible to scratching or chemical attack from household cleaners and is therefore less durable/stable than irradiated or heat treated topaz.
Can it be Produced Synthetically? What are its Imitations? Topaz can be synthesized in the laboratory but like tourmaline, this is not normally done because of an abundant supply of natural material. There is not much that is passed for topaz, except mislabeled species of gem quartz like citrine. Blue bottle glass is sometimes used as an imitation, but topaz itself is more commonly used as an imitation for other less common gemstones. What is Spodumene and What are its Basic Qualities? Spodumene is a lithium (Li)-bearing aluminosilicate, LiAlSi2O6, and is the base mineral for the gemstones kunzite and hiddenite. Naturally, spodumene is colourless to light pink and the resulting kunzite is much more common than green hiddenite. Spodumene is part of the pyroxene group of minerals, which have the general formula of ABSi 2O6 where the total cation charge of A+B must equal +4. Most pyroxene group minerals will have considerable amounts of Mg and Fe, but the geochemistry of pegmatites stabilizes this Li+ and Al+3-rich variety. Like all pyroxene group minerals spodumene forms prismatic crystals with roughly square or rectangular outlines and two distinct cleavages that run parallel to the c-axis and intersect at 90 degrees to one another. It has a hardness of 6.5 to 7 and a moderate specific gravity of ~3.2. Crystals of spodumene have been mined historically for their Li content. Specimens can reach great lengths with some up to 12.5 m. These weigh more than 50 tonnes. What is its Chemistry and Crystal Structure? Spodumene has the base formula LiAlSi2O6 with significant substitution occurring in both the Al and Li sites. Kunzite is the result of Mn+3 taking the place of Al and imparting the light pink colour, while the green colour of hiddenite is due to Cr+3 replacing Al. What Colours can Spodumene Have? How are These Colours Generated? What Gem Varieties Result? Kunzite (pink) and hiddenite (green) are the two main gem varieties of spodumene. However, some light yellow material has also been produced. Yellow spodumene typically gets lumped in with hiddenite. As with emerald, hiddenite has Cr as its chromophore. Stunning electric green specimens have come from Hiddenite, North Carolina, and the Kabul region of Afghanistan. Kunzite's colour, as with beryl's, is owed to trace amounts of manganese (Mn). This gem variety is much more common than hiddenite and is found the world over, although those originating from Minas Gerais, Brazil and the Kabul region of Afghanistan are considered premium stones. What Does it Look Like Rough? The typical tabular pyroxene shapes is quite prominent with spodumene gem varieties, as are the two cleavages associated with the pyroxene mineral group. Spodumene Recognition, Value, Treatments, and Synthetics How is it Recognized and Distinguished from Other Materials? Kunzite is easily confused with morganite (beryl), tourmaline, and sometimes topaz, amethyst, and rose quartz due to their colours. Hiddenite is mostly confused with diopside, beryl, and green glass. Refractive indices and pleochroic nature can sometimes help differentiate kunzite and hiddenite from other stones. How is Spodumene Valued? Kunzite and lower grade hiddenite are fairly common in medium to large stone sizes, and prices per carat range from ~$40 to $100 USD per carat. Of note is a 47 carat stone once belonging to Jacqueline Kenn edy Onassis that sold at a Sotheby's auction for $400,000 in 1996. Of course, the price commanded by this stone reflects its history more than its gemological worth. Because good hiddenite is much rarer than kunzite, it typically goes to collectors and its price is normally subject to availability rather than a common per carat value. What are Common Treatments for Spodumene? Like many stones, spodumene can be heat treated to eliminate undesired defects. Typically, the heating process will make faint greens and pinks more vivid. Can it be Produced Synthetically? What are its Imitations? Spodumene gem varieties can be synthesized in the laboratory but like topaz and tourmaline, this is not normally done because of an abundant supply of natural material and its "semi-precious" nature. Spodumene is a relatively low-cost gemstone, so it tends to be the imitator for other higher-end stones, such as morganite or emerald. Pink and green glass are sometimes used as imitations for spodumene, as are synthetic spinel or corundum.
Introduction
Pegmatite - the word itself sends shivers of excitement up my spine! Pegmatite is the premier rock type for finding large high quality gemstones, and with the exception of diamond, can produce basically the whole range of the most sought after coloured gems. Not all pegmatite gems occur in all pegmatites - most are restricted to specific varieties of pegmatites and some even more restricted to the type of host rock these igneous rocks intrude. Further, pegmatite gemstones tend to occur in specific locations within a pegmatite called pockets. We touched briefly on pegmatites during the beryl section, but here we'll talk a bit more about the internal structure of pegmatite and the global distribution of pegmatites (with a focus on Canada). We'll read two articles on pegmatites from Colorado and California. I also recommend reading a third paper on pegmatites in BC (reading not required). These articles are accessible for download below along with the EOSC118 guides to help you focus on the important aspects of each article. Pegmatite Genesis Recall the definition by Prof. Simmons, "pegmatite is a textural term used to describe very coarse to gigantic sized textures in intrusive igneous rocks". In addition, most pegmatites are genetically associated with larger igneous bodies and will have a base geochemical signature similar to their parental pluton. The parental pluton, commonly granite, is a key factor in the genesis of most pegmatites in that it gives rise to, or feeds, a pegmatite. During the magmatic history of a granite body it may undergo significant fractionation. Fractionation is a process that involves the sequential crystallization of minerals as granitic magma cools. As certain minerals crystallize, they essentially remove the elements required for it from the molten magma. As the magma cools further, it becomes more depleted in the elements which make up the minerals that have crystallized. An analogy would be like eating a specific colour of Smarties from a box. If you start eating only the blue ones, there will still be lots of Smarties remaining, just not many blue ones and a larger proportion of the "residual" (non -blue) Smarties In a magma, after the elements needed for the first mineral to crystallize are removed, there will still be magma remaining, just not much of the elements needed for the first crystallized mineral and a greater proportion of "residual" elements not used in that first mineral. What's left is a progressively evolved or fractionated granitic magma that is composed of the "dregs" or "residual" melt (the yellow, red, and green Smarties). What is significant is that rare elements like Be, Li, Ta, and Cs (among others) do not fit nicely into the crystal structures of the earlier crystallized minerals and consequently get strongly concentrated in this "left over" magma. This is the good stuff. When the conditions are just right, this highly evolved magma with high concentrations of rare elemen ts injects itself into the overlying host rock, forming dykes. These are normally on the order of a few meters but sometimes can be up to ~100 m across. Magmas that generate these highly fractionated pegmatites are often called fertile while those that are not are called barren. Pegmatites originating from fertile granite will often show geographic zoning of rare metal enrichment. Geochemical Families The high concentrations of rare element and the resulting mineral assemblages facilitate the classification of pegmatites. The concept of geochemical families for pegmatites was recognized many years ago, but the most commonly used scheme was introduced by Cerny in 1991. In Cerny's scheme, pegmatites are divided into four main groups based on three main factors: 1. depth of emplacement below the surface; 2. range of temperature; and 3. type of rare element enrichment These three variables control what mineral phases can be present in a given pegmatite since mineral phases will only be stable in specific conditions. Based on the variables above, the four pegmatite groups are: 1. Abyssal (high temperature, variable pressure); 2. Muscovite (low T, high P); 3. Rare-element (low T, low P); and 4. Miarolitic (medium T, low P) Of these four groups, rare-element pegmatites tend to produce the most gemstones. This group is further divided into two categories, Lithium-Cesium-Tantalum ("LCT") and Niobium-Yttrium-Fluorine ("NYF") based on the dominant rare elements. Between the two subdivisions, LCT pegmatites give rise to the most gem miner als. All pegmatites contain large amounts of gases and volatiles that are effective fluxes for the pegmatite magma. Fluxes are elements and/or compounds that reduce the freezing point of the magma. Lower freezing points result in more time for crystal growth. Fluxes also decrease nucleation, which result in fewer crystals, and increase movement of elements to where crystals are growing, which result in bigger crystals. In many pegmatites, the collection of fluxing agents include H2O, F, Cl, carbonate (CO3)-2, borate (BO3)-3, Li, and phosphate (PO4)-2.
Pegmatite Morphology Pegmatites are often described using mineral and structural zonation features. A zone is defined as a regions within a pegmatite with a common or regular set of minerals and textures. These zones tend to form concentrically but not necessarily evenly. Simple pegmatites have homogeneous textures and simple mineral assemblages throughout the igneous body, showing no segregation into discrete zones. These types of pegmatites tend to have many crystals of smaller size rather than a small number of larger sized crystals. The simple mineralogy and small grain size limits their gem potential. Zoned pegmatites are heterogeneous, differentiated and exciting! They consist of a "core" zone surrounded by distinct zones moving outwards through core margin, intermediate, wall and finally the border zone. These types of pegmatites are often symmetrical in cross section but will show irregular 3D shapes when the pegmatite is considered in full. Thicknesses of the different zones depend on the individual pegmatite and the pegmatite field and crystal sizes in general coarsen towards the core. The border zones consist of finer grained crystals comprising feldspars and quartz with the occasional tourmaline or garnet. Wall zones are medium to coarse grained and also consist primarily of feldspars and quartz; minerals such as garnet and beryl start to appear here. The intermediate zone is where we start to see very coarse crystals and also where significant gemstones begin to appear. In this zone, the mineralogy is dominated by feldspars but the tourmaline variety starts to change from schorl to elbaite. The core margin is a nucleating site for gem minerals that will eventually grow unhindered into the core zone w here pockets exist. Extremely coarse gem quality crystals are common here and typically include beryl, spodumene (kunzite), elbaite, and other rare element minerals. Inwards from the core margin is the core itself, which is most commonly composed of quartz. However, this is the most common region where pockets develop and where the best crystals grow! These regions are usually fluid -filled close to the end of a pegmatite's life. This environment allows crystals that have started to grow on the core margin to extend as far as they can into this pocket. Truly magnificent specimens from pegmatites usually originate from pockets. Many pegmatologists love to say that you haven't lived until you've unearthed a pocket zone! Complex pegmatites are zoned pegmatites that have been altered from their original concentrically zoned form by further influx of evolved fluids or magma with high volatile content (e.g., H 2O, B, (PO4)-2, F). Often, this overprint will be of either LCT or NYF geochemical character. Textures in these pegmatites will include all those in the zoned pegmatites, but many minerals are partially or fully destroyed from corrosion, while new mineral assemblages are stabilized. These pegmatites also produce fantastic mineral specimens and they also tend to be the best type for rare metal ore deposits. Corrosion The abundance of volatiles associated with highly fractionated magmas can unfortunately be detrimental for early stage gem minerals in pegmatites. When these volatile elements are present at the end of a pegmatite's life, the geochemical environment that they create may be corrosive to earlier formed minerals. As a result, the early minerals can be partially or completely corroded and replaced with minerals of similar composition, but greater stabilit y under these late-stage conditions. A common example of this is beryl, which is sometimes found in resorbed "bullet" shapes alongside other beryllium -rich minerals such as bertrandite. Spodumene often gives way to lepidolite, and tourmaline gives way to clays and micas. Corrosion also leads to the development of pegmatite pockets, however, since the fluids are very corrosive they tend not to produce significant amounts of gem material. Why is it Rare? Gem bearing pegmatites are rare for a number of compounding reasons. Firstly, these require a geological environment with relatively abundant granitic rocks and where magmas have a chance to evolve and fractionate to the point where a rare-element enriched body is generated. Furthermore, the material from whi ch the parent magmas are produced also needs to be fertile for rare element enrichment. Ideally, this magma is released from the parent granite and emplaced in sufficiently wide enough dykes that encourage pocket growth. High volatile concentrations are necessary to facilitate growth of crystals, but not too high that a corrosive environment that would destroy many crystals is created. For gem minerals to be preserved, pegmatites need to be in a tectonic environment that will allow them to be brought upwards into the crust while not allowing the rocks they are hosted in to deform too much, otherwise crystals may become cracked or broken. And finally, the slow and steady work of erosion is required to remove enough overlying rock that pegmatites can be found on the surface, either in secondary alluvial deposits or from primary sources. How Big Does it Get?
Pegmatites consistently produce the largest gemstones of any rock type. Beryl crystals over 1 m in length are common and some of the largest specimens have been on the order of 18 m! Large gem quality stones from pegmatites include heliodor (up to 2,000 carats), aquamarine (the "Marta Rocha" weighs ~75 lbs), morganite (the "Rose of Maine" weighed more than 50 lbs when it was first uncovered), tourmaline (Paraba variety up to ~50 carats), spodumene crystals over 10 m in length, and topaz crystals over 200 lbs. Where is it Found and Mined Globally? Pegmatites are found across the globe, but high quality gem bearing pegmatites are much rarer. Famous localities include Brazil, Madagascar, Russia, Pakistan, and the United States. Other notable regions include Italy, Mozambique, Namibia, and Afghanistan. The best gem pegmatites in Brazil, and arguably the world, occur in a region called Minas Gerais. It is located ~500 km north of Rio de Janeiro and consists of a very prolific pegmatite region. The following map is modified from Proctor (1984) and shows the distribution of pegmatites and their primary gem content. In the United States, the Pala District of California has produced some of the most magnificent rubellite tourmaline, in addition to top quality morganite and aquamarine in the world. Read this article by Mark Mauthner published in Rocks & Minerals, which details the discovery of the "49er Pocket" where hundreds of gem specimens, from good to very fine, have been recovered. Use this Mauthner-2008 Reading Guide to help you with the article. In Colorado are pegmatites of the Mt. Antero region just southwest of Denver. This area is known for some of the best aquamarine crystals in North America. Bryan Lees, in his 2005 article in Rocks & Minerals, describes the excavation of a gem bearing pocket from this region. Read the article and use this Lees-2005 Reading Guide to help you through it. Another notable location in the U.S. is the Mount Mica mine in Maine. This area has produced beautiful material since the early 1800's. Where is it Found Locally? Canada is host to numerous pegmatites, but none yet with gems like those seen in Brazil or the U.S. Pegmatites occur in the relatively young Cordillera of Western Canada (BC, Yukon, NWT), within the Canadian Shield that makes up the majority of Canada's landmass, and on the eastern margin in the Appalachians. Prof. Cerny of the Department of Geological Sciences at the University of Manitoba has been studying pegmatites of the Canadian Shield for decades. His publication on the subject in 1990 includes an extensive list of pegmatite sites between Manitoba and eastern Canada. In the Canadian Shield, the "Superior Province" is likely the best area for finding significant sources of gem bearing pegmatites. Within this area is Tanco Pegmatite Mine in southeastern Manitoba, an extremely evolved pegmatite that has been mined for its Li, Cs, and Ta ores for several decades. L. Groat of the UBC Department of Earth and Ocean Sciences has been studying pegmatites in the younger Cordilleran rocks on the western side of the country. His publication in 1996 summarizes studies performed on a suite existing in Yukon and Northwest Territories. Extensive pegmatite fields also exist in B.C. and many have been located in the Kootenay area. In his 2003 article, J. Brown investigated the potential of pegmatite-related mineralization in this area with a focus on about a dozen individual occurrences. There are a number of topaz and beryl bearing granites and pegmatites in the Canadian Appalachians, including the Brazil Lake Pegmatite and the coarse grained East Kemptville leucogranite. Notable pegmatite -associated gem beryl occurrences include emerald at Ghost Lake in Ontario, goshenite at the Little Nahanni Pegmatite Group, and aquamarine throughout the Kootenays. Topaz associated in pegmatites have been located in Yukon Territory near the towns of Teslin and Swift River. Tourmaline of the elbaite variety has been found in pegmatite from Northwest Territories and Quebec. Most exciting is gemmy tourmaline from the O'Grady Batholith, a remote area at the top end of the Nahanni River. There, multi coloured crystals have been recovered up to 10 cm long and include pink, brown, green, orange, and blue colours. Other localities closer to Yellowknife include the Ryder pegmatite and a number of sporadic smaller pegmatite bodies. In Quebec, pegmatites near the Leduc mine are producing gem quality tourmaline, normally of pink and green colour. So far, no large gem quality stones have been produced from Canada's pegmatites although some small er material of high quality have been cut. With such a diverse geological framework in Canada under many different environments for pegmatites, it is only a matter of time until some fantastic finds are uncovered. Cabochon and Carved Gems Many of the gemstones that are fashioned into cabochons or carved do not readily form transparent crystals appropriate for faceting. However, they typically show vibrant colours or interesting textures that have been valued both in antiquity (e.g., lapis lazuli) and today (e.g., jade). Lapis Lazuli Text Content: pp 244 - 245 Sodalite
Text Content: p 243 Lapis lazuli is actually not a single mineral, but rather a mixture comprising mostly of lazurite, pyrite, and calcite with minor diopside, sodalite and hayne. It has a long history dating back to the Egyptian and Babylonian civilizations and in historical Europe was often called "ultramarine". Today, most lapis lazuli is produced from Sar-e-Sang, Afghanistan, with minor production in the Lake Baikal region of Russia and the Andes in Chile near Coquimbo. Canada hosts one known lapis lazuli deposit, and it is located in the far north of Baffin Island. The USA has two main localities: one at Italian Mountain in Colorado and the other near Balmat, New York. Jade: Jadeite and Nephrite Text Content: pp 274 - 275, 280 - 281 Tremolite and Actinolite Text Content: p 279 The term "jade" in today's usage refers to two different and highly valued translucent rocks: jadeite and nephrite. These are gems of high durability and a wide range in colours (not just the familiar green). Historically, the term "jade", including it's various names in different cultures, referred to a number of durable stones such as serpentinite, that had similar uses as jade but were of different mineralogical compositions. Mineralogically, nephrite is actually a mass consisting primarily of finely crystallized (microcrystalline to cryptocrystalline) amphibole with a composition between tremolite to actinolite (Ca2(Mg,Fe)5Si8O22(OH)2). Jadeite is a rock (i.e., polymineralic) comprised primarily of the pyroxene mineral jadeite (NaAlSi2O6). Jadeite is often described as having a granular texture and nephrite a silky texture, a direct result of the fibrous nature o f amphibole and blocky texture of pyroxene. The translucency of jade is often evaluated by the maximum thickness required to allow significant light through the stone. Translucency is sometimes called the ventana (Spanish for window). Jade has been treasured by many cultures, perhaps most notably the Chinese and the Maori of New Zealand. The earliest record of jade being used as tools dates back almost 5,000 years in Asia and Europe. Artifacts dating much further back have also been found. There are many occurrences of jade in the world and they appear on every continent. British Columbia produces a large amount of nephrite jade and exports considerable amounts, especially from the Cry Lake and Dease Lake regions. Although there are many showings and occurrences throughout the province, other significant region s include Cassiar, Mount Ogden, and Bridge River. Turquoise Text Content: pp 196 - 197) Similar to lapis lazuli, turquoise has been used throughout antiquity as a valuable carving and cabochon stone. It is a complex Cu - Al phosphate mineral and usually forms in microcrystalline masses with other accessory minerals, such as malachite, chrysocolla, and iron oxides. Deposits of turquoise form near the surface close to Cu -bearing intrusive rocks (e.g., porphyries) as a result of surface waters percolating to depth and interacting with these Cu-rich rocks. Some of the more famous deposits are in Egypt, Iran, and the United States. Egyptian turquoise deposits played an important role in ancient civilizations but have long been exhausted. Persian turquoise, long considered the best in the market, have deposits there are running out. In North America, the turquoise deposits of New Mexico (e.g., Cerrillos Hills) and Arizona (Bisbee), which are associated with Cu ore deposits, have been mined by the Indians of the Am erican Southwest and traded with Aztec tribes for many years. There are no known turquoise occurrences in Canada. Other Precious Gemstones Quartz Gems Text Content: pp 219 - 233 The base formula of the quartz group of gems is SiO2, but the ubiquity of this base mineral group and the large number of variations give rise to no less than a dozen gem varieties. The most precious of the group is opal; other popular varieties include amethyst, citrine, and agate. Quartz crystal forms range from giant euhedral crystals of quartz in pegmatites (up to ~6 m long and 1.5 m across) down to cryptocrystalline varieties like agates where it would be tough to find individual crystals even under a microscope. Because of the commonness of these gems and their global distribution, they have been used by most of the world's civilizations and cultures in some way or form. Their "upper intermediate" hardness (Mohs = 7) makes quartz harder than many materials, but soft enough that it could be carved and fashioned efficiently. Many carved quartz artifacts are dotted throughout antiquity and include basins, bottles, boxes, rings, cameos, statues, beads, and skulls. The pages in the textbook will guide you through the wonderful diversity of quartz's gem varieties. Chrysoberyl (var. alexandrite)
Text Content: p 159 Alexandrite is a very valuable Cr-bearing variety of chrysoberyl. It is particularly remarkable as a gemstone because of its colour change properties and strong pleochroism. Under normal daylight, good quality alexandrite will display a vivid emerald green colour. Under incandescent light (tungsten filament) it will display a strong purple -red colouration. The reason for this pleochroism is its large absorption band centered in the yellow region (thus allowing greens and some red) as a result of the Cr content and the effect of the different emission spectra of light sources. Daylight emits strongly in the blues and greens in the visible range, while incandescent light is strongest in the red region of the visible range. The end result is the "colour change" effect that alexandrite is famous for. Stones with strong colour change effects are very rare and rarely reach sizes above 10 carats. Gem specimens with strong colour change can easily fetch up to $10,000 USD per carat, even those of smaller sizes. The most famous region for alexandrite are the Ural Mountains in Russia. However, good quality stones have also come from Brazil, India, Myanmar, and Tanzania. Although alexandrite has not been discovered in North America, its parent mineral, chrysoberyl has been noted in eastern USA (Maine, New York, Connecticut) and in various pegmatite localities in central and western US, including northeastern Washington State. Garnet Group Text Content: pp 300 - 303 The garnet group is composed of six main minerals divided into two main mineral series, ugrandite and pyralspite. The series names are taken from the minerals included in each group: UGRANDite for Uvarovite, GRossular, and ANDradite and PYRALSPite for PYRope, ALmandine, and SPessartine. All of the ugrandite series garnets contain essential Ca in the structure, while those of the pyralspite series require Al in their structure. The minerals of the garnet group show a wide range of colour from "ruby red" to "emerald green" and als o have good hardness. Garnet itself is quite a common mineral, but the gem varieties are uncommon to rare, with tsavorite and green demantoid garnets fetching up to ~$3,000 USD per carat for stones under 3 carats. Olivine (var. peridot) Text Content: pp 298 - 299 Like zoisite, many people may not familiar with olivine, but the gem name peridot is as common as a neighborhood cat. The word "peridot" is likely derived from the Arabic word for gem, "faridat". Historically, the best gem peridot comes from St. John's Island in the Red Sea (once called Topazios, now called Zabargad) where mining of this gem stretches back over 3,500 years. Peridot is quite common in smaller sizes and occurs prolifically in peridotite xenoliths within basalts around the world. Of note is a beach in Hawaii that is comprised almost entirely of olivine called Papakolea Beach but is locally referred to as the "Green Sand Beach". Large stones like those from Zabargad, however, are quite rare. The largest cut fine peridot gem weighs just over 300 carats and today sits at the National Museum of Natural History. Organic Gems Gems also originate from organic material and play an important role in the gem and jewellery industry. Many of these gems have the same qualities and characteristics that mineral-based gemstones have and deserve their acknowledgment as genuine gemstones. The three most common organic gems are pearl, amber, and coral. Other organic gems include ivory, fossilized bone, mother of pearl, copal, and wood. Pearls Text Content: pp 322 - 325 Pearls are natural concretions that form inside mollusks in response to an aggravating foreign particle. This occurs naturally but very rarely. The development of the cultured pearl industry was thought to have started in China during the 13th century but only produced blister pearls. In response to rising demands and decreasing global stock, techniques for growing full sphere cultured pearls were developed in the late 19th century by Japanese entrepreneurs. Today, nearly 100% of pearls are cultured. Amber Text Content: pp 314 - 315 Most amber comes from Russia, Poland, and the Dominican Republic. Amber is fossilized coniferous tree resin that dates back almost 345 million years ago. Most of the amber of the Baltic region, however, is younger a t ~45 million years old. This material is thought to have been used since the 13th millennium BC both as jewellery and later as fishing buoys due to its low density. One of the exciting features of amber is its ability to preserve insects and material from its original growth many millions of years ago ( the basis of the story for the movie Jurassic Park!). Ants, spiders, millipedes, and wasps, are among the many critters that have been found in amber, but lizards probably top the list in rarity! Coral Text Content: p 320
Coral is an organic gemstone that is unfortunately associated with a lot of controversy. It comes in many different colours, forms beautiful patterns, and can take a high polish. These qualities and its associ ation with the ocean have made coral a very popular material and which, at the same time, have put the coral reefs of the world under significant stress. As a result, a movement to stop the harvesting of coral and introduce synthetic material has been gain ing momentum since 2000. Tiffany & Co Foundation has provided significant funding for these activities, with strong support from a number of international organizations.