Most gemstones show some color shift under different light conditions and this is always apparent when we change between incandescent, fluorescent, and daylights for photographing different kinds of gemstones. And when we visit gem shows, we always see a variety of lights that dealers use to make their gemstones look their best. But few stones actually show a dramatic change in color as a result of exposure under different kinds of lighting environments.
The most important of these is of course alexandrite. Chrysoberyl is the species name and alexandrite is the variety name. Alexandrite is that variety of chrysoberyl that changes color as a function of the light source; green in daylight and red under incandescent light.
Several other well known gemstones, including diaspore, sapphire, garnet and spinel may also change color as a function of the light source but the color change of top alexandrites is distinctive and attractive under any light conditions.
Spinel sometimes show a blue to violet color change but the change is usually weak and the stones never look like alexandrite. Occasionally, we’ll find a spinel with a strong color change and we once had one that showed and impressive change from red to blue.
Turkish diaspore is being sold under the name of Zultanite and higher quality versions of this stone shift from varying shades of green in daylight to a pinkish brown under incandescent light.
But only sapphires and garnets can show any real resemblance to alexandrite.
Color change sapphires are basically known to occur in two types; the ones that change from blue to purple and the ones that change from green to red. The stones that change from green to red are the ones that can be confused with alexandrite and they only occur at the deposit in Songea, Tanzania. Although the daylight colors are usually somewhat muddy, they can be very red under incandescent light and do look similar to some of the alexandrites from the same country. Because of their similarity to alexandrites, they are referred to as alex type sapphires in Japan.
However, it is the color change garnets, especially the ones from Bekily in Madagascar and some of the stones from Nandagala, Tanzania and Chavia, in Kenya that most resemble alexandrite. The stones are actually a mixture of pyrope and spessartite and can show several colors depending on the light source. Although they look a lot like alexandrites they are different because they change color throughout the day. They are green or blue grey in the early morning and reddish in the late afternoon or in strong sunlight. Some of the stones are almost blue especially under fluorescent light but most of them are grey blue or green in daylight and change to red under incandescent or late afternoon light. The stones can show an excellent color change and can easily be confused with alexandrite.
Without gemological tests, the stones can be distinguished from alexandrites by the needle like inclusions that are common in some of them or by the way the stones change color according to the time of day. Although they look like alexandrites, these garnets will appear red in the afternoon while the alexandrites remain green. Although the color change of some of these garnets is intense and equal to the color change of top quality alexandrites, they are available for a fraction of the price of alexandrites and demand for them is always strong.