Gems & Gemology, Winter 1982, Vol. 18, No. 4
Gemstones with Alexandrite Effect
E. Gübelin and K. Schmetzer
The term alexandrite effect refers to the apparent change of color in certain minerals from blue-green or greenish violet in daylight to red or reddish violet in incandescent light. This effect was discovered in chrome-bearing chrysoberyl from the Ural Mountains as early as the beginning of the 19th century. In more recent times, it has also been observed in certain varieties of garnet, corundum, spinel, kyanite, fluorite, and monazite. It has been determined that the absorption spectrum of all alexandrite-like minerals is characterized by transmission maxima in the blue-green and red regions and by a transmission minimum in the yellow region. The color of minerals with two such regions of transmission is determined in daylight (richer in blue and green) by the position of the transmission maxima and in incandescent light (richer in red) by the ratio of transmission in the red and transmission in the blue-violet sections of the visible spectrum.