Lecture 4: Color in Gemstones
I. What is Color
For some gems, color is the most important factor in determining value. Certain
gemstones have nearly immutable color, while others vary so endlessly the reasons
behind their coloration remain a mystery. To understand the phenomenon of color, one
must be familiar with the concept of light. Since this is an intermediate level course, I
expect you to understand both the wave and particle behaviors of light. Without this
knowledge, any further discussion of color is difficult at best.
Visible light is only a small part of the electromagnetic spectrum. Radio waves, light
waves, and cosmic rays make up a continuous spectrum of energy. Radio waves have
wavelengths up to a billion times longer than visible light, while Cosmic rays have
wavelengths a billion times shorter than visible light. Visible light occupies
approximately the middle of the spectrum from 400 to 700 nanometers (billionths of a
). Each color has its own range of wavelengths. Red light occupies 630-700
nm, while Violet light has wavelengths of 400-430 nm. If you've ever seen a rainbow,
you've seen the seven basic colors of light: Red, Orange, Yellow, Green, Blue, Indigo,
Electromagnetic Radiation is a continuous spectrum of energy from long-wavelength
radio waves to very, very short-wavelength cosmic rays. Note that this is not to scale.
Our perception of color occurs when specific of wavelengths of light strike certain cells
on the back of our eyes. For instance, if light of 700nm wavelength were to hit our eyes,
we would perceive it as red light, just as we would perceive 400nm wavelengths to be
violet. You should also note that light from several wavelengths together form other
colors. All wavelengths together (as you should already be aware) form white light, while
no light is black. Note, however, that this is not quite the same principle as additive
mixing in paint pigments.
If an object absorbs most wavelengths of light and reflects only specific ones back to our
eyes, the object will appear to be that color. Thus if a gem reflects light of 700-630 nm, it
will appear red (and so on). Herein lies the most important concept behind color in
Gems are different colors because they reflect specific wavelengths of light.
Here the violet colored gemstone is reflecting violet light wavelengths, while absorbing
all the others.
You should also remember my comment from last week: a blue sapphire will appear
black in candle light, because candle light contains many red wavelengths, but not much