Alexandrite: Properties, Origins, Treatments and Value

Alexandrite is chrysoberyl that has taken up chromium, and chromium in chrysoberyl does something it does nowhere else in the trade: it leaves the stone balanced so exactly between green and red that the light in the room decides which colour you see. By a north window it is green. By a candle it is red. The trade has a phrase for it, emerald by day and ruby by night, and for once the phrase undersells the stone, because a fine alexandrite is rarer than either.

What it is

Chrysoberyl is BeAl₂O₄, beryllium aluminium oxide, crystallising in the orthorhombic system, frequently as twinned crystals with a six-sided outline. Hardness 8.5, below only corundum and diamond, with good toughness and no troublesome cleavage. Refractive index 1.746 to 1.755, birefringence around 0.009, specific gravity 3.73. Strongly pleochroic, showing green, orange and purplish red down its three directions, which is part of what the cutter has to manage. Ordinary chrysoberyl is yellow to green and honest; the cat’s-eye variety, with parallel needles giving a sharp band of light, is prized on its own; alexandrite is the chromium-bearing variety, and cat’s-eye alexandrite, combining both effects, is close to unobtainable in fine quality.

Where the colour change comes from

Chromium, Cr³⁺, replacing aluminium, as in ruby and emerald. What differs is the crystal field around it. In chrysoberyl the chromium absorbs strongly in the yellow, leaving two windows of transmission of nearly equal strength, one in the green and one in the red. Daylight, balanced toward the blue and green, pushes the eye to the green window; incandescent light, heavy in red, pushes it to the red. The stone does not change. The illumination does, and the stone shows whichever half of its transmission the light favours. The strength of the effect depends on chromium concentration and on how much iron is present to muddy it, which is why origin shows in the quality of the change. Judging any stone in the seller’s lamp is dangerous; with alexandrite it is half the examination, and the other half is taking the stone to the window.

Where it comes from

The Urals, in Russia, gave the stone its name and its legend: found in the emerald mines near Yekaterinburg in the 1830s and named for the future tsar, with a green-to-red change that remains the reference. The deposits were effectively exhausted long ago and Russian stones trade as antiques with a provenance premium. Sri Lanka has produced alexandrite for centuries, often larger but with a change that runs from yellowish green to brownish red. Brazil, since the Hematita find in Minas Gerais in 1987, set the modern standard, with stones whose change rivals the Urals. India, in Andhra Pradesh and Orissa, produces commercial material; Tanzania at Tunduru, Madagascar and Zimbabwe contribute. Origin can be stated by a laboratory from inclusions and chemistry and, as always, it is an opinion.

How it is judged

The change first, because it is the stone’s reason to exist. A fine alexandrite goes from a clean bluish green in daylight to a clean purplish red under incandescent light, with neither colour carrying brown or grey. The trade speaks of the percentage of change, meaning how completely the stone shifts; a strong change of a hundred percent with poor colours is worth less than a somewhat weaker change between two beautiful ones. Then saturation in each light, on the usual three axes. Then clarity, where alexandrite is often included and a clean stone commands a premium. Then cut, which must manage strong pleochroism and show the change face-up. Then size, and the curve is savage: fine alexandrite above three carats is rare, above five is historic, and most of what trades is under two.

Treatments, and what to demand on paper

Nothing in the standard repertoire improves alexandrite. It is not heated, not diffused, not filled in any routine way, which makes natural material unusually honest goods and puts all the risk on one question: is it natural. Synthetic alexandrite has been grown by flux and pulled methods for decades, with a convincing change and inclusions that fool the unwary. Colour-change synthetic corundum and synthetic spinel, cheap and everywhere, are sold as alexandrite in every market town in the world; the first shows a blue-to-purple shift and the second a grey-to-mauve one, and neither is close to the real thing in daylight. A report from a serious house states identity first, and with this stone identity is nearly the whole examination. Origin, where it carries a premium, is the second line.

The market

Alexandrite has no auction record book of consequence and no colour appellation, because supply is too thin to build either. It is priced stone by stone. The finest Brazilian and Russian stones of size, with strong clean change and Swiss or GIA paper, trade per carat at levels comparable to fine unheated sapphire, and the top of the range simply does not come to market in most years. Below that, Sri Lankan and Indian material with weaker change fills the commercial tiers, and cat’s-eye alexandrite of quality is priced as the individual object it is. It is a collector’s stone and a specialist’s, and its liquidity reflects that.

Care

Hardness 8.5 and good toughness make alexandrite one of the most wearable stones in the trade. It takes ultrasonic cleaning, daily wear and ordinary jeweller’s handling. There is no treatment to protect and no oil to dry out. The only care it needs is an owner who looks at it in two lights.

Where I stand

Alexandrite is not a stone I hold in quantity, because almost nobody does; it is a stone I hunt on mandate for the collector who wants the rarest object in the case rather than the most famous. The examination is the one I use for everything, doubled: colour in daylight against masters, then colour under a lamp against a second set, then the laboratory for the identity line before anyone pays. A fine one is worth the year it takes to find. A false one is the most common alexandrite in the world.

Questions collectors ask

Why does alexandrite change colour?

Chromium in chrysoberyl absorbs light in a way that leaves the stone transmitting almost equally in green and red. Daylight is rich in green, so the eye sees green; incandescent light is rich in red, so the eye sees red. The stone does not change; the light does, and the stone shows whichever half of its window the light favours.

Is alexandrite treated?

No commercial treatment improves it, which makes natural alexandrite unusually honest goods. The risk is synthetic: flux-grown and pulled crystals have been made for decades with a convincing change, and colour-change synthetic corundum and spinel are sold as alexandrite in every market town. Identity on a laboratory report is the first line to read.

What makes an alexandrite valuable?

The strength and quality of the change first: a clean green to a clean red, without brown or grey in either. Then saturation in both lights, then clarity, then size, where anything above three carats of fine change is rare and above five is historic. Russian origin carries a premium for history; Brazilian stones set the modern standard.