Alexandrite
Chrysoberyl · also called Colour-change chrysoberyl
Alexandrite is the colour-change variety of chrysoberyl: green to bluish-green in daylight, red to purplish-red under incandescent light. The change is caused by chromium. Discovered in the Urals in the 1830s, it is now mined chiefly in Brazil, Sri Lanka and East Africa, and remains among the rarest gems in commerce.
What alexandrite is
Alexandrite is chrysoberyl, beryllium aluminium oxide, in which a small amount of chromium has replaced aluminium in the structure. Ordinary chrysoberyl is yellow to greenish; the chromium substitution produces something no other common gem does — a stone that is a different colour in daylight than under a filament lamp.
It was found in the Urals in the 1830s and named for the Russian heir apparent. The Russian deposit produced the material against which everything is still judged and was effectively exhausted within decades, which is why the historic stones carry their own market.
Why the colour changes
Chromium in chrysoberyl absorbs strongly in the yellow part of the spectrum and leaves two transmission windows open: one in the blue-green, one in the red. Daylight is rich in the blue end, so the green window wins and the stone reads green. Incandescent light is rich in the red end, so the red window wins and the stone reads red.
Nothing changes inside the stone — what changes is what the light offers it. The gem is a filter, and the effect is real physics rather than an optical trick, which is why it works reliably and why it cannot be faked by cutting or setting.
How a change is graded
Three things are judged, and they matter in this order.
The completeness of the change. Fine material moves fully from green to red. Most commercial material shifts from a greyish or brownish green to a purplish or brownish red, and the trade describes the change as a percentage — 100 per cent meaning a full turn.
The quality of the two colours. A stone can change completely and still be unattractive at both ends. Both the green and the red must be worth looking at on their own.
Clarity and size. Alexandrite above one carat with a strong change is scarce; above three carats it is a collector's stone at any quality.
Judge it under real daylight and a real incandescent bulb. Modern LED lighting sits between the two and will show you a muddle rather than a change.
Where it comes from, and what origin does to price
Russian material carries a mythic premium and almost never appears. Brazil, particularly the Hematita deposit found in 1987, produced the finest material of the modern era and is largely worked out. Sri Lanka yields larger stones, often with a change toward khaki and raspberry rather than green and red. Tanzania and India supply much of the commercial goods.
As with padparadscha, origin reports are worth commissioning above a certain value, because the premium attached to a named source is large enough to attract misattribution.
What is sold as alexandrite and is not
This is where buyers lose money, and it is rarely a treatment problem.
Synthetic alexandrite has been grown commercially since the 1970s by flux and pulling methods. It is real chrysoberyl with real colour change, and separating it from natural material takes a laboratory reading the inclusions and the trace elements.
More common still is colour-change synthetic corundum, sold for decades as an alexandrite substitute. It turns from a greyish blue-mauve to a purple-red and fools people constantly — but its refractive index of about 1.76 is nowhere near chrysoberyl's 1.744–1.755, and its specific gravity near 4.00 is far above 3.70–3.78. A refractometer and a balance separate them in a minute.
Colour-change garnet, spinel and fluorite also exist, are natural, and are legitimate stones under their own names.
What this means if you are buying
See the stone yourself under both light sources before agreeing a price, and distrust photographs entirely: the two-panel image showing a perfect green and a perfect red is the most manipulated picture in the gem trade.
Above a carat, ask for a report from a first-tier laboratory that states natural chrysoberyl, describes the change, and gives the origin. Below that value, at least confirm the refractive index and specific gravity yourself — those two numbers alone eliminate the substitute that catches most people.
See our gems and rough, or send us a brief.
What colour is alexandrite?
Green to bluish-green in daylight and red to purplish-red under incandescent light. The change is caused by chromium, which leaves two transmission windows open in the spectrum. Which window dominates depends on whether the light source is rich in blue or in red.
Why is alexandrite so rare?
It requires beryllium and chromium in the same rock, and those elements normally occur in geologically incompatible settings. The original Ural deposit was exhausted within decades, and the finest modern source, Hematita in Brazil, is largely worked out. Fine material above one carat is genuinely scarce.
How do you tell natural from synthetic alexandrite?
Synthetic chrysoberyl needs a laboratory, which reads the inclusions and trace elements. Colour-change synthetic corundum, the more common substitute, is easy: its refractive index is about 1.76 against chrysoberyl's 1.744–1.755, and its specific gravity is near 4.00 against 3.70–3.78.
Does alexandrite change colour under LED light?
Poorly. LED lighting sits between daylight and incandescent in spectral content, so it tends to show a muddled intermediate colour rather than a clean change. Judge the stone under real daylight and a real filament bulb, which are the two conditions the effect is defined by.
Is Russian alexandrite worth more?
Substantially, when the origin is documented. The Ural deposit is the historic reference and produces almost nothing today, so material with a credible Russian origin carries a large premium. That premium is big enough to attract misattribution, which is why an origin report matters above a certain value.
Alexandrite available now
Nothing in alexandrite is listed at the moment. We buy continuously — tell us the colour, the weight range and the budget and we will come back when something matches.
Send a briefLooking to buy alexandrite? See the collection, the rough, or send us a brief.