Geo331 lect3

Geo331 lect3 - Lecture 3: Physical Properties of Gemstones...

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Lecture 3: Physical Properties of Gemstones I. Hardness: Mohs' useful scale Hardness is determined by a mineral's resistance to scratching. There are two measures of hardness: scratch hardness and indentation hardness. Generally, we use the scratch hardness which is based on a rank from 1 (softest) to 10 (hardest). If we compare, for example, diamond and quartz (the main component of most beach sands) we will find that quartz crystals are readily scratched by diamond but diamonds can not be scratched by quartz. From this we can infer that diamond is harder than quartz. Mohs' scale of hardness ranks ten minerals in order of differing hardness: 10 Diamond (hardest) 9 Corundum 8 Topaz 7 Quartz 6 Feldspar 5 Apatite 4 Fluorite 3 Calcite 2 Gypsum 1 Talc (softest) Diamond crystal. Photo courtesy FJ Barlow mineral collection, MH photo. Commonly available materials have a corresponding hardness, and can be used to determine the hardness of an unknown sample. 6.5 steel file 6 normal glass 5.5 pocket knife 3 copper coin 2 fingernail Looking at Mohs' scale, and remembering back to the distribution of elements in the earth's crust, Quartz (Silicon and Oxygen: SiO2) is extremely abundant. Most dust and sand contains quartz. Thus, to resist abrasion from sand and dust, most durable gems should be harder than quartz. Unfortunately for jewelry, most minerals with hardness greater than 7 on Mohs' hardness scale are brittle. II. Cleavage and Fracture: Minerals can break by fracture (like bottle glass) or by cleaving. The term 'cleavage' refers to the tendency of a mineral to break or split along planes of weakness in a crystal. The planes that crystals are most prone to split along are planes where atomic bonding is weakest. Despite their extreme hardness, Diamonds are prone to splitting along specific cleavage planes.
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Calcite (left) and the cleavage fragments after being hit with a hammer (right). Quartz showing "conchoidal" fracture. Note also the glassy luster. Malleability and Ductility Malleable relates to the property to bend. Ductile relates to being able to be drawn into a thin wire. Metals are malleable and ductile - most gemstones are too brittle and do not have these properties. III. Specific Gravity: Specific gravity refers to the mass of a mineral compared to the mass of an equal volume of water. If a mineral has the same mass as an equal volume of water, it has a specific gravity (SG) of 1. Something that weighs less than an equal volume of water would have an SG of less than one (and would also float). Most minerals have a specific gravity higher than one. Diamonds have an SG of 3.52 and platinum (one of the most dense materials on earth) has an SG of 21.5. Specific gravity is closely related to density. The more dense a material, the higher its specific gravity.
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Geo331 lect3 - Lecture 3: Physical Properties of Gemstones...

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