Fascinating Phenomena in Gemstones, Part 4
Written by Brian Wilson On February 18, 2020.
The first 3 parts of this article can be found here, here, and here.
Continuing on in our look at phenomenal gemstones,
today we examine one of the favorite phenomena – Color Change This is part 4 in a reprint of an article from the American Gem Society seen here.
A small number of gemstones display the color change optical phenomena. Depending on the lighting environment, the color change appearance can vary due to the shifting wavelengths. The technical term for this is photochromism or photochroism; “color-change” is a lot easier to say!
The best-known color changing gemstone is alexandrite. When viewed in sunlight, it appears greenish. When placed under incandescent light, it appears reddish. Other varieties of color-changing gemstones include sapphire, garnet, spinel, diaspore, and tourmaline.
On the website, GIA (Gemological Institute of Amercia), has the following to say,
Often described by gem aficionados as “emerald by day, ruby by night,” alexandrite is the very rare color-change variety of the mineral chrysoberyl. Originally discovered in Russia’s Ural Mountains in the 1830s, it’s now found in Sri Lanka, East Africa, and Brazil, but fine material is exceptionally rare and valuable.
Color Change Sapphires
On the website, GIA (Gemological Institute of Amercia), has the following to say about the quality of sapphires as related to color change,
Color-change sapphires are corundum’s chameleons—stones that change color under different lighting. Under daylight equivalent (fluorescent or LED daylight-balanced) light, the typical color-change sapphire’s basic color ranges from blue to violet. Under incandescent light, it ranges from violetish purple to strongly reddish purple. Some rare color-change sapphires change from green in daylight to reddish brown in incandescent light.
When gem experts judge color-change sapphires, they describe the color change as weak, moderate, or strong. The strength of the stone’s color change is the most important quality factor affecting its value.