Gemstones that exhibit unusual optical effects are known as phenomenal gems.These effects include chatoyancy (the cat's eye effect), asterism (the star effect), adularescence (the shimmering effect, as in moonstone), play of color (as in opal) and iridescence (rainbow-like colors, as in labradorite). These remarkable phenomena have been known for centuries, and gems that exhibit them are especially valuable.
But more recently there is another optical effect that has drawn a lot of attention in the market -- color change. These are gemstones that display different colors according to changes in lighting, usually displaying a different color under incandscent light from that displayed under natural or fluorescent light.
The most famous of these is the rare and expensive alexandrite, a variety of chrysoberyl. But color-change garnets and color-change sapphires have appeared on the market that show equally dramatic color changes.
Color change sapphire, for example exhibits a range of different color changes, ranging from pink/purple to green/purple. Color change garnet changes from bronze in daylight to a rose pink under incandescent light.
Why is it that only a few gemstones, such as certain natural sapphires, exhibit these dramatic color changes, since nearly all gemstone color depends on the nature of the illumination? The answer is that these color change gems have two approximately equal sized transmission windows. A red gemstone appears red because it absorbs all frequencies of light except for red. A gemstone that absorbs both blue and red light will appear blue when the light is rich in blue wavelengths.
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