Gem library
Alexandrite
Emerald by day, ruby by night
Alexandrite is the chromium-bearing variety of chrysoberyl, and the most celebrated colour-change gem: green in daylight, raspberry red by candlelight or under an incandescent lamp. That metamorphosis, combined with extreme rarity, makes it one of the most expensive gems in the world.
- Hardness (Mohs)
- 8.5
- Density
- 3.70 – 3.78
- Crystal system
- Orthorhombic
- Refractive index
- 1.745 – 1.759
- Cleavage
- Imperfect on {110}
Mineralogical identity
- Family
- Chrysoberyl — oxide group
- Species
- Chrysoberyl
- Variety
- Alexandrite
- Chemical formula
- BeAl₂O₄
- Crystal system
- Orthorhombic
- Classification
- Precious stone
- Rarity
- Very rare
- Indicative price
- €2,000 – 30,000/ct
Etymology
Described in 1830 in the Urals and named after Tsarevich Alexander, the future Alexander II of Russia, who came of age the year it was found. Green and red were the colours of the Russian Empire.
Physical & optical properties
- Hardness (Mohs)
- 8.5
- Density
- 3.70 – 3.78
- Refractive index
- 1.745 – 1.759
- Birefringence
- 0.008 – 0.010
- Cleavage
- Imperfect on {110}
- Fracture
- Conchoidal to uneven
- Lustre
- Vitreous
- Diaphaneity
- Transparent
- UV fluorescence
- Weak red
- Pleochroism
- Distinct
- Colours
- green, red, purple, multicolour
Formation, geology & history
The colour change, explained
Alexandrite owes everything to chromium, which replaces part of the aluminium in the chrysoberyl structure. That chromium absorbs strongly in the yellow, leaving two nearly equivalent transmission windows: one in the blue-green, one in the red. The eye then decides according to the available light. Under daylight, rich in blue, it reads green; under an incandescent lamp or a flame, rich in red, it reads red. The phenomenon is not pleochroism — which depends on viewing angle and also exists in alexandrite — but a metameric effect, called the «alexandrite effect» by extension to every gem that shows it.
Formation and rarity
Beryllium and chromium almost never occur in the same rocks: the first belongs to granitic pegmatites, the second to ultrabasic mantle rocks. A very particular geological situation is therefore required — the contact between a pegmatite and a serpentinised ultrabasic rock — for the two to meet. That is exactly what happened in the emerald mines of Tokovaya in the Urals, and at Hematita in Brazil. This chemical improbability explains alexandrite's rarity far better than geography: stones over three carats with a clean change are so few that in a good year they are counted in dozens.
Quality, price and pitfalls
An alexandrite's value is judged first on the amplitude of the change — a «100 %» stone goes from clear green to purplish red, a «50 %» merely from greyish green to brownish red. Then saturation, then clarity, and finally weight, which counts enormously: beyond three carats, price per carat soars. Historic Russian stones, often small, carry a provenance premium. The market is riddled with imitations, starting with the colour-change synthetic corundum sold since the 1900s as «alexandrite» — a stone that shifts from grey-blue to violet, and has nothing to do with it.
How to recognise it
- The two-light testLook at the stone by a window, then under an incandescent bulb or a candle flame. An alexandrite swings from green to raspberry red. A cool white LED will not work: the source must be genuinely rich in red.
- Hardness 8.5Chrysoberyl scratches quartz and topaz effortlessly and is barely scratched by corundum. A colour-change stone that scratches easily is not alexandrite.
- Three-colour pleochroismOn top of the colour change, alexandrite shows green, yellow-orange and purplish red along its axes. This trichroism accompanies genuine chrysoberyl and is missing from the synthetic corundums sold under its name.
Imitations, treatments & confusions
- Synthetic «alexandrite» corundumThe historic imitation, still found in antique jewellery. This vanadium-bearing synthetic sapphire shifts from grey-blue to mauve-violet, never green to red, and its density of 4.0 gives it away.
- Flux- or pull-grown synthetic alexandriteChemically identical to natural and visually very convincing — it is legitimately sold as synthetic. Only characteristic flux inclusions or laboratory analysis settle the matter. At this price level a certificate is essential.
- Colour-change garnet and spinelSome garnets and spinels genuinely shift from blue-green to purple. They are fine stones, but an order of magnitude cheaper: check density and refractive index before accepting the name alexandrite.
Deposits & origins
Russia (Urals, Tokovaya mines — the historic deposit, effectively exhausted), Brazil (Hematita, Minas Gerais — the best modern source), Sri Lanka (large stones, more modest change), Tanzania (Tunduru), India (Andhra Pradesh), Madagascar, Zimbabwe
Care & precautions
- Warm soapy water and a soft brush: hardness 8.5 and no sharp cleavage make alexandrite very easy to clean.
- Ultrasonic and steam are generally tolerated on a sound stone, but avoid them if surface-reaching inclusions or fractures are visible.
- No sensitivity to light or water: this is a gem for daily wear, one of the toughest on the market.
- Store separately all the same: only diamond, corundum and moissanite can scratch it, but it will scratch everything else.
Traditions & crystal healing
- Chakras
- Heart · solar plexus · crown
- Zodiac signs
- Gemini · Scorpio
- Element
- Fire · Air
- Month
- June
Too rare and too recent for a folk tradition, alexandrite was read in Russia as an imperial omen, and contemporary crystal healing sees in it the stone of accepted duality: it is said to help reconcile opposites within oneself, move through metamorphosis and align intuition with reason. It is the official stone of the fifty-fifth wedding anniversary.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
Why is alexandrite so expensive?
Because it requires the improbable meeting of beryllium and chromium, and because stones with a clean change at attractive sizes are extraordinarily few. A well-coloured three-carat natural alexandrite routinely exceeds the price of an equivalent diamond.
How can a synthetic alexandrite be identified?
Usually not by eye: it is chemically identical. A laboratory identifies flux inclusions and growth features specific to synthesis. Above a few hundred euros, insist on a certificate from a recognised laboratory.
Do all alexandrites change colour that much?
No, and that is the first price criterion. Brazilian and Russian stones offer the cleanest change; Sri Lankan and Indian ones often stay in an olive green turning brownish red, far less spectacular and much cheaper.