Alexandrite

Alexandrite is the colour-change variety of chrysoberyl, originally discovered in the Ural Mountains of Russia in the 1830s and named after Tsar Alexander II. The optical phenomenon is produced by the substitution of chromium ions in the crystal lattice: the crystal absorbs light in the yellow region of the spectrum and transmits both red and green. In daylight (rich in blue-green light) the stone appears green; under incandescent light (rich in red wavelengths) it shifts to purplish red. With the Russian deposits largely worked out, the finest modern alexandrites come from Brazil and Madagascar.

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