Alexandrite

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. 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.

Strictly speaking, the color of most gemstones appear differently under changing light conditions. It is well-known that sapphire only shows its true color in natural light, and that's why the weekend gemstone market here in Chanthaburi takes place in the open air during daylight hours only. While blue sapphire suffers under incandescent lighting, the color of both ruby and yellow sapphire are enhanced under such light. That's because incandescent lighting has more energy in the warm (orange red yellow) end of the spectrum.

Color Change Garnet

Many of these color differences are apparent only to the trained eye. But the so-called color change gems exhibit this tendency to such a dramatic degree that it is obvious to even the untrained eye. Alexandrite, for example, appears green in daylight and purplish under incandescent lighting. Color change garnet changes from bronze in daylight to a rose pink under incandescent light. Color change diaspore is usually a kiwi-green under natural light, and a brownish pink to raspberry in candlelight. Color change sapphire exhibits a range of different color changes, ranging from pink/purple to green/purple.

So why is it that only a few gemstones 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 (e.g., fluorescent light) and red when the light is rich in red wavelengths (e.g., incandescent lighting).