Often described by gem aficionados as “emerald by day, ruby by night,” alexandrite is the very rare color-change variety of the mineral chrysoberyl. Originally discovered in Russia’s Ural Mountains in the 1830s, it’s now found in Sri Lanka, East Africa, and Brazil, but fine material is exceptionally rare and valuable.
1830
The year alexandrite was discovered in Russia’s Ural Mountains.
Czar Alexander II
Gem’s namesake who emancipated Russia’s serfs and was assassinated in 1881.
580nm
This absorption band allows alexandrite to shift from red to green when viewed under different light sources.
Facts
- Mineral: Chrysoberyl
- Chemistry: BeAl2O4
- Color: Bluish green in daylight, purplish red in incandescent light
- Refractive Index: 1.746 to 1.755
- Birefringence: 0.008 to 0.010
- Specific Gravity: 3.73
- Mohs Hardness: 8.5
Alexandrite
The alexandrite variety displays a color change dependent upon the nature of ambient lighting. Alexandrite results from small scale replacement of aluminium by chromium ions in the crystal structure, which causes intense absorption of light over a narrow range of wavelengths in the yellow region (520–620 nm) of the visible light spectrum. Because human vision is most sensitive to green light and least sensitive to red light, alexandrite appears greenish in daylight where the full spectrum of visible light is present, and reddish in incandescent light which emits less green and blue light. This color change is independent of any change of hue with viewing direction through the crystal that would arise from pleochroism.
Alexandrite from the Ural Mountains in Russia can be green by daylight and red by incandescent light. Other varieties of alexandrite may be yellowish or pink in daylight and a columbine or raspberry red by incandescent light.
Stones that show a dramatic color change and strong colors (e.g., red-to-green) are rare and sought-after, but stones that show less distinct colors (e.g. yellowish green changing to brownish yellow) may also be considered “alexandrite” by gem labs such as the Gemological Institute of America.
According to a popular but controversial story, alexandrite was discovered by the Finnish mineralogist Nils Gustaf Nordenskiöld (1792–1866), and named alexandrite in honor of the future Emperor of All Russia Alexander II Romanov. Nordenskiöld’s initial discovery occurred as a result of an examination of a newly found mineral sample he had received from Perovskii, which he identified as emerald at first. The first emerald mine had been opened in 1831. However, recent research suggests that the stone was discovered by Yakov Kokovin.
Alexandrite 5 carats (1,000 mg) and larger were traditionally thought to be found only in the Ural Mountains, but have since been found in larger sizes in Brazil. Other deposits are located in India (Andhra Pradesh), Madagascar, Tanzania and Sri Lanka. Alexandrite in sizes over three carats are very rare. Today, several labs can produce synthetic lab-grown stones with the same chemical and physical properties as natural alexandrite. Several methods can produce flux-grown alexandrite, Czochralski (or pulled) alexandrite, and hydrothermally-produced alexandrite. Flux-grown gems are fairly difficult to distinguish from natural alexandrite as they contain inclusions that seem natural. Czochralski or pulled alexandrite is easier to identify because it is very clean and contains curved striations visible under magnification. Although the color change in pulled stones can be from blue to red, the color change does not truly resemble that of natural alexandrite from any deposit. Hydrothermal lab-grown alexandrite has identical physical and chemical properties to real alexandrite.
Some gemstones falsely described as lab-grown synthetic alexandrite are actually corundum laced with trace elements (e.g., vanadium) or color-change spinel and are not actually chrysoberyl. As a result, they would be more accurately described as simulated alexandrite rather than “synthetic”. This alexandrite-like sapphire material has been around for almost 100 years and shows a characteristic purple-mauve colour change, which does not really look like alexandrite because there is never any green.