“Emerald by day, Ruby by night”
With its chameleon-like qualities, Alexandrite is a rare variety of the mineral chrysoberyl. Its color can be a lovely green in daylight or fluorescent light, changing to brownish or purplish-red in the incandescent light from a lamp or candle flame. Which is a result of the complex way the mineral absorbs light. Its because incandescent light contains less green and blue spectrum. The color change is due to chromium atoms replacing the aluminum in the chrysoberyl structure, which causes intense light absorption over a narrow range of wavelengths.
Although chrysoberyl crystals are not uncommon, the gemstone variety is one of the rarest and most expensive gems globally, with specimens seldom exceeding 10 carats. In addition, it has the extraordinary visual property of appearing green in daylight but red under tungsten light; other chrysoberyl forms occur in green, greenish-yellow, and yellow.
Significant alexandrite deposits were first discovered in 1830 in Russia’s Ural Mountains. The gem was named after the young Alexander II (1818–1881), heir apparent to the throne. The rare gem caught the country’s attention because its red and green colors mirrored the national military colors of imperial Russia. You can find beautiful specimens now in Sri Lanka, East Africa, and Brazil, but fine material is scarce and valuable.
Alexandrite is the birthstone for June and the gem for the 55th wedding anniversary. When certain types of long, thin inclusions are oriented parallel to each other in this June birthstone, they can create another phenomenon called chatoyancy or the cat’s-eye effect. Few gems are as fascinating – or as stunning – as cat’s-eye alexandrite.
Famous Alexandrite Gems
The largest known faceted alexandrite, a 65.7-ct green/red color change stone from Sri Lanka, resides at the Smithsonian Institution. The most significant Russian gems weigh about 30 carats. However, the vast majority of alexandrites weigh under one carat. Stones over five carats are scarce, especially with good color change.
Following are the other alexandrites of notable sizes:
- British Museum of Natural History (London): 43 and 27.5 carats (Sri Lanka).
- Institute of Mines (St. Petersburg, Russia): a cluster of three crystals, 6 x 3 cm (Urals)
- Fersman Museum (Moscow, Russia): crystal group, 25 x 15 cm, crystals up to 6 x 3 cm (Urals).