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Color Change Gems
Gemstones that display unique optical effects have always held a special fascination for gem collectors. In gemology these are known as phenomenal gems because of their special optical phenomena. The most famous examples include star sapphires and star rubies, and chrysoberyl cat's eye.
One of the lesser known, but nonetheless interesting, examples is gems that display different colors under different lighting. This phenomenon is quite unusual, and ranges from a subtle color change to quite a dramatic effect. The most notable example is alexandrite, a rare form of chrysoberyl that displays a strong color change. But there are a few other gem varieties as well that also exhibit this phenomenon.
It is important to distinguish color change from what is sometimes called color shift. Many gemstones will show some degree of color shift under different lighting conditions. Blue sapphire, for example, looks more blue under natural light than under incandescent lighting because natural light is richer in blue wavelengths. The color of ruby, on the other hand, is enhanced by incandescent lighting, which is weighted more heavily to frequencies in the red and yellow part of the spectrum. We should also distinguish color change from gems such as tourmaline which are pleochroic, meaning they display different colors when viewed from different angles under the same lighting.
But gemstones which are classified as color change gems display a change which is more pronounced then color shifting, and they display their change only when the quality of the lighting changes. Alexandrite, for example, usually appears green in natural light and a reddish-purple under incandescent lighting.
In addition to alexandrite, color change specimens can be found in sapphire, garnet, Turkish diaspore and fluorite. Color change garnet displays a deep green or blue green under natural light and red or purple under incandescent light. We have recently seen some fine specimens in these from Kenya and Madagascar, and some of them display a strong color change that rivals top grade alexandrite. Color change sapphire exhibits a range of different color changes, ranging from pink/purple to green/purple. These are occasionally found in sapphire sources around the world, but the recent examples we have seen come from Madagascar and Tanzania.
Fluorite is a fairly common mineral known for its huge variety of colors. But there is a rare color change fluorite from Brazil that changes color from a topaz-like blue in natural light to an amethyst-like purple in incandescent light. Diaspore is a recent find in Turkey that displays a more subtle color change, from a light-green or golden in natural light to a pinkish brown in low light conditions such as candelight.
The color change phenomenon is due to properties of the gem's crystal lattice. A gemstone appears red because it absorbs all frequencies of light except for red. When a gem has a low absorption in some part of the spectrum, this is referred to as its transmission window. Some rare gems have multiple approximately equal sized transmission windows, and they will exhibit a color change when the light changes. So a gemstone that absorbs all frequencies except for green and red will appear green when the light is richer in green wavelengths and red when the light is red in red wavelengths.
Because color-change gems are so rare, prices tend to be higher than for non-color-change specimens of the same variety. But if you are a collector or just want to own a unique and beautiful rarity of nature, color-change stones make wonderful jewelry.