“Green in sunlight. Red in lamplight. Color-changing alexandrite is nature’s magic trick.” ~Gemological Institute of America
Photograph by David Weinberg, via Wikimedia Commons
Alexandrite is a very rare variety of the chrysoberyl mineral family. It is highly prized for its color-changing properties, which famously cause it to appear emerald-green in daylight and ruby-red by night. Depending on the individual stone and lighting conditions, it can also display hues of blue-green and purple (particularly under incandescent light). Some more common stones will display more of a yellow-green in daylight and brown-red under incandescent light.
A small-scale replacement of aluminum by chromium ions is both what distinguishes Alexandrite from other chrysoberyl stones and causes its color-changing properties. The unique crystalline structure is responsible for intense absorption of light over a narrow wavelength of the yellow region of the visible light spectrum. The stones appear green in daylight, when the full spectrum of visible light is present, because the human eye is more sensitive to green, and more reddish under incandescent light because such artificial light sources tend to produce less green and blue light.
In addition to its color-changing properties, it is also possible for a “cat’s eye” to form in some samples, meaning that the gem can exhibit two rare phenomena in a single gem.
Discovered in the 1830s, Alexandrite is named after Tsar Alexander II Romanov, though there is some controversy as to whether it was first identified by Finnish mineralogist Nils Gustaf Nordenskiöld or Yakov Kokovin, a 19th century mine manager in Russia’s Ural Mountains who has gained favor in recent years. The GIA credits the gem’s discovery more generally to miners in the 1830s due to this murky history.
While it is still extremely rare, deposits of Alexandrite have since been discovered in areas outside of the Urals. These include sites in Brazil, Madagascar, Sri Lanka, Tanzania, and the Andhra Pradesh region of India. Large 5 carat samples of the stone were once thought only to occur in Russia, but similarly sized samples have since been found in Brazilian mines. Even with the context of the extreme scarcity of this unique stone, sizes of 3 carats are still exceedingly rare.
Prized for both its beauty and its scarcity, Alexandrite can be imitated by both synthetic and simulated lab methods, but a qualified gemologist with the right tools can distinguish between naturally occurring and artificial stones.
Alexandrite is the birthstone of June, and is also the traditional gem for 55th wedding anniversaries.