These gems dramatically change colors before our eyes, depending on the light
In the realm of phenomenal gemstones, those that can change color have always drawn the interest of consumers. Some gems can change from deep purplish blue to raspberry red in an instant in different lighting conditions, others from greenish to yellow or from gray to red.
Color-change stones include some rare sapphires and garnets, as well as some spinels, fluorites, alexandrites and a few synthetics and manmade glass. Gemologists often refer to the phenomenon as the alexandrite effect in reference to the best-known color-change gemstone.
Alexandrites are named after Czar Alexander II of Russia, who is said to have come of age on the day alexandrites were discovered in the countrys Ural Mountains in 1830. The gems change from reddish to green, the colors of Imperial Russia. Alexandrites are found also in Brazil, Madagascar, Sri Lanka and Tanzania.
Color-change garnets are rare and relatively small, but new finds in Madagascar and East Africa have popularized them once again. They tend to change from smoky grayish blue to vibrant raspberry red.
The causes for color change are somewhat complex. Color-change gemstones have the property to change color according to the nature of light they are seen in: incandescent (orange-to-red-rich light), fluorescent light and daylight (blue rich light), says Laurent Massi of the University of Nantes in Nantes, France, a doctoral student and color-change expert.
The color change is due to two crucial factors: the nature of the light used for the observation and the ultraviolet visible absorption spectra of the stone, he says. Those spectra correlate to the presence of a trace element in the stone, such as chromium, vanadium and, in some cases, rare earth elements such as neodymium.
In many cases, absorption spectra in the visible spectrum show two main transmission windows and two main absorptions. According to the nature of the light, one transmission window is favored, and the gemstone color observed is the result of this selection. For instance, the chromiums spectrum in alexandrite shows two transmission windows at 490nm (blue green) and at 650nm (red). In daylight, blue rich, the first absorption is favored so the stone appears green or bluish green.
This effect is helped by the fact the human eye is very sensitive to green radiation color. Under an incandescent light, the second transmission window is favored so the stone appears red.
The strength of the color change is key. The purer the green in alexandrite, the better, though the red must be pure also and as devoid of other color influences as possible. Many alexandrites can have muddy colors under different lights, which is not as pleasing.
Prices for fine alexandrites can soar to astronomical levels ($8,000 to $14,000 per carat wholesale) for gems above 3 carats. Color-change garnets, meanwhile, are even more rare than color-change alexandrites but can be acquired for a fraction of those prices.
by Robert Weldon, G.G.
Photo by Robert Weldon.
||Under fluorescent lights or daylight, this color-change garnet becomes smoky blue (left). Once its returned to a room with incandescent lighting, the color reverts to deep reddish purple.