Within this voluminous world of gemstones are those mercurial species which exhibit color change when viewed in different light sources. This is not the same as pleochroism, where a stone shows different colors when viewed down different crystal axes, such as iolite and Andalusite, but an optical phenomenon where a gemstone exhibits a color difference over the whole stone when viewed in different light sources.
Typically one color is seen in incandescent light which is rich in red wavelengths, and another in fluorescent or natural daylight sources which are rich in the blue end of the spectrum. The most famous gem of this type, and the one to which all others are generally compared, is Alexandrite (the color change variety of the species chrysoberyl). The finest specimens from old Russian sources, and virtually unobtainable in today's market, switch from near ruby red to near emerald green with a change in the light source. Present sources of Alexandrite, which ranges from poor to good in quality are India, Sri Lanka, Madgascar, and South America.
Under certain conditions, when a gemstone contains an element which reacts very selectively and strongly to red wavelengths (such as chromium), different body colors can be produced due to the richness of reds in incandescent light and their relative scarcity in either daylight or fluorescent light sources. Depending on the other elements present, various combinations of colors and strengths of the effect will be seen.
The three species most usually sold in color change forms are: chrysoberyl, sapphire, and garnet. Many other gemstones may show the effect to a greater or lesser degree including: diaspore, tourmaline, spinel, iolite, and beryl. There are many global sources for these stones such as Turkey, Brazil, and various African countries.
Synthetic color change stones have been available nearly as long as synthetics themselves. First among these was color change synthetic corundum marketed as "Alexandrium" with true synthetic Alexandrite being a more recent addition.
Fine Russian Alexandrite is at the apex of all color change stones in terms of both quality and value. Alexandrite from other sources such as Brazil, India, and Sri Lanka varies in price depending primarily on the saturation of the colors and the strength of the change.
Other color change species are available at more modest prices, which for fine sapphires might reach into the same range as non-Russian Alexandrite, with the best grades of color change garnet somewhat lower. No established price ranges for most species are found as specimens are few and generally go to collectors.
Size, clarity, and color determine the value of these gemstones as well as completeness of the color change and the saturation of each color. A stone with a modest color change having two saturated and attractive colors may be as valuable or more valuable than one whose change is more dramatic but whose colors are greyed or browned.
Our thanks to our Gemology Expert, Barbara W. Smigel, PhD, Graduate Gemologist, GIA for her contribution to this article.
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