One of the most interesting of the optical phenomena to be seen in gemstones is color change. This characteristic is sometimes confused with pleochroism, where a stone shows different colors when viewed down different crystal axes, such as in Andalusite and iolite. True color change, however, exhibits itself as a color difference over the whole stone when viewed in different light sources.
Typically one color is seen in incandescent light, a source 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, which are 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. (The faceted stones below are from Sri Lanka, and the cat'seyes from India).
Under certain conditions, when a gemstone contains a chromophore 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.
Metamerism is the general phenomenon of two colored object appearing to be slightly different hues depending on the light source, and it is by no means confined to gemstones. Although almost every colored gemstone will look slightly different indoors and outside, but we only call it "color change" when the difference is realtively dramatic. Metamerism of this subtle type is a factor that a jeweler must to take into account when attempting to match gems for a setting or in finding a replacement for a missing gem. Customers can be very unhappy when the gems match perfectly indoors but not outdoors or vice versa.
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.
As in all gems, size, clarity and color affect value, but with this group there are two additional factors: completeness of the color change and the attractiveness/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.