She had inherited the ring from her grandmother who had brought it from Russia in the 1920s. The stone in the centre was modest in size, perhaps 1.5 carats, in a simple gold bezel. She had been told it was "a special green stone." She wore it occasionally without particular interest until a colleague, a gemologist visiting from London, asked to look at it more carefully. The colleague held it near the window. Green, clearly. Vivid, slightly blue-green, the kind of green that catches attention. Then the colleague held the same ring under the incandescent bulb of the desk lamp. The stone went red. Not reddish, not brownish. Red. The colleague said, very quietly, "Do you know what this is?" She did not. "This is Ural alexandrite. This is from the original deposit. Stones like this haven't come out of that ground in nearly a hundred years." The stone that had been "a special green stone" for three generations was a piece of geological history: a chromium-bearing chrysoberyl from the Ural Mountains that had been discovered, mined, celebrated as the finest colour-change gem in the world, and then essentially exhausted before the Russian Revolution made the records of its existence inaccessible to the wider world for decades.
Quick answer: what is alexandrite? Alexandrite is the colour-change variety of chrysoberyl (beryllium aluminium oxide, BeAl₂O₄) coloured by chromium (Cr³⁺). It appears green to blue-green in daylight (which is rich in green wavelengths) and red to purplish-red in incandescent light (which is rich in red wavelengths). The colour change is a direct result of chromium's specific transmission window in the chrysoberyl crystal field, which allows both green and red light to pass while absorbing yellow and blue, making the dominant apparent colour dependent on the light source's spectral composition. Strong colour-change alexandrite from the original Ural Russian deposits is the rarest and most valuable material of this type; Brazilian alexandrite from the Hematita mine area is the most significant modern commercial source. Sources: Nassau, K., American Mineralogist 63 (1978); GIA Gem Reference Guide (2006), pp. 54–57; Schmetzer, K., Russian Alexandrites, Schweizerbart Science Publishers (2010).

The colour change: what it looks like and how to assess it

The colour change in alexandrite is assessed on a scale from weak (barely perceptible) to strong (complete and dramatic reversal). The most commercially valuable material shows a strong change from a vivid, pure green or blue-green in daylight to a vivid red or purplish-red in incandescent light. The change must be clean in both lighting conditions: not greenish-brown in daylight and brownish-red in incandescent, but genuinely green and genuinely red.

The standard method for assessing colour change is to hold the stone alternately near a north-facing window in natural daylight and under a tungsten incandescent bulb. Fluorescent lighting and LED lighting do not produce the full red because they lack the red-wavelength richness of incandescent light. The dramatic effect that makes alexandrite memorable is specific to incandescent illumination (GIA Gem Reference Guide, 2006, pp. 54–57; Wise, R.W., Secrets of the Gem Trade, 2016, pp. 142–150).

Alexandrite colour change: daylight vs incandescent Daylight (balanced spectrum) Chromium absorbs red + blue; transmits GREEN GREEN Vivid green, blue-green Incandescent (red-rich spectrum) Red wavelengths dominate; RED transmitted RED Red to purplish-red Source: Nassau, K., American Mineralogist 63 (1978); GIA Gem Reference Guide (2006). Same stone, same chromium, opposite dominant colour.

Alexandrite colour change: the same stone appears green in balanced daylight (chromium transmits the green portion) and red in red-rich incandescent light (the abundant red wavelengths pass through the same transmission window). Source: Nassau (1978); GIA Gem Reference Guide (2006).

Why alexandrite changes colour: the chromium transmission window

The colour change mechanism is one of the most elegant examples of crystal field physics in gemology. Chromium (Cr³⁺) in chrysoberyl's crystal field produces a specific absorption pattern: it strongly absorbs yellow and blue wavelengths, leaving two transmission windows, one in the green (approximately 550–600nm) and one in the red (approximately 680–700nm). In most lighting conditions, one window dominates the apparent colour (Nassau, K., American Mineralogist, 63:219–229, 1978; Fritsch, E. and Rossman, G.R., Gems and Gemology, 24(2):81–102, 1988).

In daylight, the solar spectrum has a roughly even distribution across visible wavelengths with a slight green peak. More of the stone's green transmission window is activated than the red, making the stone appear green. In incandescent light, the tungsten filament produces a spectrum strongly weighted toward red and near-infrared wavelengths. More of the stone's red transmission window is now activated than the green. The stone flips to red. The underlying physics has not changed; only the illuminant's spectral composition has changed, and the chromium's specific double transmission window exploits this difference with a completeness no other gem chromophore achieves (Nassau, 1978; Fritsch and Rossman, 1988; Schmetzer, K., Russian Alexandrites, 2010).

The chrysoberyl family: alexandrite's relatives

Chrysoberyl (BeAl₂O₄, beryllium aluminium oxide) is a distinct mineral from beryl (Be₃Al₂Si₆O₁₈) despite sharing beryllium and aluminium in their formulas. Chrysoberyl is orthorhombic; beryl is hexagonal. They are related geochemically but not structurally. The chrysoberyl family has three gem varieties:

Ordinary chrysoberyl: Yellow to yellowish-green chrysoberyl without colour change, coloured by iron. Not alexandrite; not particularly valuable. Used in commercial jewellery.

Cat's eye chrysoberyl (cymophane): Chrysoberyl showing chatoyancy, a silky moving band of light caused by parallel inclusions of fibrous rutile or hollow channels oriented along the crystal's length. When cut as a cabochon with the inclusions perpendicular to the dome, the reflected light concentrates into a line that moves as the stone is rotated. Cat's eye chrysoberyl is the finest chatoyant gem. The "cat's eye" of the gem trade, without qualification, refers specifically to this stone (GIA Gem Reference Guide, 2006, pp. 54–57).

Alexandrite: Colour-change chrysoberyl, coloured by chromium. When alexandrite also shows chatoyancy, it is called alexandrite cat's eye and is among the rarest gems in commercial production (GIA; Wise, 2016).

The chrysoberyl family: BeAl₂O₄, three gem varieties Chrysoberyl Fe³⁺ colourant Yellow to yellow-green No colour change Commercial grade Cat's eye (cymophane) Fe³⁺ + fibrous inclusions Silky chatoyancy Finest chatoyant gem Cabochon cut Alexandrite Cr³⁺ colourant Green/red colour change Most valuable variety Rarest commercial gem

The chrysoberyl family: ordinary chrysoberyl (iron-coloured, yellow-green), cat's eye chrysoberyl (chatoyancy from fibrous inclusions), and alexandrite (chromium colour change). All three have the same base chemistry BeAl₂O₄. Source: GIA Gem Reference Guide (2006); Wise (2016).

Russian Ural alexandrite: the original and the finest

The Ural Mountains of Russia, specifically the deposits near Yekaterinburg (formerly Sverdlovsk) in the emerald-mining belt that also produced Russian emerald, are the source of the finest alexandrite ever recovered. The deposit was discovered in 1830 or 1834 (sources vary on the exact date) in the emerald-bearing mica schist formations that characterise the eastern Ural gem belt. The stone was named in honour of Tsar Alexander II, who reached his majority around the time of its discovery, and quickly became the most prestigious gem in the Russian imperial court (Schmetzer, K., Russian Alexandrites, Schweizerbart Science Publishers, 2010, pp. 10–25; Wise, 2016, pp. 142–146).

What makes Russian alexandrite different

The best Ural alexandrite shows a colour change that is both complete and vivid: a pure, strongly saturated green in daylight (not olive, not grey-green, not blue-green diluted with grey) changing to a pure, strongly saturated red in incandescent light (not brownish-red, not purplish-brown). The specific combination is referred to in the trade as "emerald by day, ruby by night", an overstatement, since no alexandrite matches the finest emerald green or pigeon-blood ruby red, but an accurate characterisation of the direction and completeness of the change. This specific quality, vivid green changing to vivid red with minimal brown, grey, or olive intermediate, is extremely rare in alexandrite from any source and essentially defines the Ural material at its finest (Schmetzer, 2010; Wise, 2016).

Current status of Ural alexandrite

The Ural deposit has been largely exhausted of fine gem-quality material. Small quantities of alexandrite continue to be recovered from the emerald-producing region, but the output of stones with the finest colour change is minimal and sporadic. Fine Russian alexandrite currently in the market comes from historical collections, estate sales, and the dismantling of Russian imperial-era jewellery, not from new production. This absolute supply constraint mirrors the Kashmir sapphire situation and is a structural driver of the investment value of documented Russian material (Schmetzer, 2010; Wise, 2016; Christie's Geneva).

Brazilian alexandrite: the modern commercial source

Brazil's Hematita mine area in Minas Gerais state is the most significant modern commercial source for alexandrite. Brazilian alexandrite has been produced since the mid-20th century and ranges from commercial-grade material with weak colour change to fine stones with strong change approaching (but not matching) the finest Russian material. The colour character differs from Russian: Brazilian alexandrite typically shows a blue-green in daylight rather than pure green, and a more purplish or brownish-red in incandescent light rather than pure red (Wise, 2016, pp. 146–148; GIA Gems and Gemology Brazilian alexandrite research).

The market's assessment: fine Brazilian alexandrite with strong, clean colour change is a genuinely beautiful and rare gem that commands significant prices, but it does not achieve the premiums of fine Russian material of equivalent size because the colour character is different and the Russian origin is historically unique. For buyers who want strong-change alexandrite at more accessible price points than the finest Russian, fine Brazilian material with a major laboratory certificate is the practical route (Wise, 2016; AGL; GIA).

Indian alexandrite: Andhra Pradesh and beyond

India is a significant producer of alexandrite from deposits in Andhra Pradesh, particularly in the Narasimhapur area, and from the Orissa state deposits. Indian alexandrite represents a large proportion of the commercial-grade material available in the global market, and the Indian market is a major consumer of alexandrite for Jyotish purposes, alexandrite is considered a substitute for natural cat's eye chrysoberyl (Lehsunia) in some Jyotish traditions, though the two are different stones and should not be conflated (GIA India; Schmetzer, 2010; GJEPC).

Indian alexandrite quality ranges widely. The finest Indian material shows good colour change, often with a slightly brownish intermediate in some lighting conditions. Commercial-grade Indian alexandrite, which is the bulk of production, shows weak to moderate change. Understanding the colour change strength assessment is essential for navigating the Indian alexandrite market (GIA; AGL; Wise, 2016).

Assessing strength of colour change

The strength of colour change in alexandrite is the primary quality variable after colour quality in each lighting condition. Laboratories and the trade generally use four grades:

Alexandrite colour change strength: commercial grades Weak Under 20% change Barely perceptible Most Indian commercial Low value premium Moderate 20–50% change Clearly visible Commercial baseline Some Indian, some Brazil Strong 50–80% change Dramatic and clear Fine Brazilian; some Russian Fine gem tier Excellent 80–100% change Complete vivid reversal Finest Russian; rare Brazil Investment/collector tier "Emerald by day, ruby by night" Source: GIA Colored Stone assessment; Wise, Secrets of the Gem Trade (2016), pp. 142–150; AGL colour change grading.

Alexandrite colour change strength grades. The commercial baseline is moderate change; fine gem quality requires strong to excellent change. The "excellent" designation, with a complete and vivid reversal from green to red, is the characteristic of the finest Ural Russian material and the rarest Brazilian stones. Source: GIA; Wise (2016); AGL.

Synthetic alexandrite: the most significant fraud risk

Synthetic alexandrite is produced commercially by both the Czochralski (flux) and hydrothermal methods. It has the same chemical composition as natural alexandrite, identical hardness (Mohs 8.5), identical refractive index, and often shows a stronger, more vivid colour change than most natural material because the laboratory can control the chromium concentration precisely. Synthetic alexandrite is sold at Rs 200–800 per carat; natural alexandrite of fine quality starts at Rs 50,000 per carat and goes to Rs 10,00,000+ per carat for fine Russian material. The fraud margin is enormous (Nassau, K., Gems Made by Man, 1980; GIA Colored Stone identification).

The key diagnostic features distinguishing synthetic from natural alexandrite are microscopic: natural alexandrite from the Urals contains characteristic inclusions including silk (rutile needles), fingerprint fractures, and specific crystal inclusions that are products of the geological environment. Synthetic Czochralski material shows curved striae and gas bubbles. Hydrothermal synthetic shows its own specific growth features. A trained gemologist with a microscope identifies the distinction routinely; the naked eye and refractometer cannot (GIA; AGL; Schmetzer, 2010; Nassau, 1980).

In the Indian Jyotish market, much of the material sold as Vaidurya (alexandrite or cat's eye) is synthetic. Any alexandrite purchase for Jyotish use requires a GIA India or AGL certificate confirming natural chrysoberyl before the investment is made (GIA India; AGL; Behari, B., Gems and Astrology, 1991).

Frequently asked questions

Is alexandrite the rarest commercial gem?

For practical purposes, yes. Fine natural alexandrite with strong colour change is more difficult to source in commercial quantities than Kashmir sapphire, unheated Burmese ruby, or Colombian emerald. The Ural Russian deposit is essentially exhausted of fine material; Brazilian production at the finest colour-change level is limited; and the combination of size, colour quality in both lighting conditions, and strength of change that defines fine alexandrite is genuinely rare. GIA identifies it as one of the rarest coloured gems in commerce. The caveat: "commercial gem" excludes museum-piece minerals that are rarer but not traded; by practical market availability, alexandrite at the fine quality tier is the rarest stone most buyers will ever encounter (GIA; Wise, 2016, pp. 141–142).

Does alexandrite change colour under all types of light?

No. The colour change requires a light source with sufficient red-wavelength richness relative to green. Daylight and fluorescent or LED lighting produce the green appearance. Incandescent (tungsten filament) light and candlelight produce the red. Many modern LED lights, even warm-white ones, do not produce a complete red response because they lack the continuous red-wavelength emission of incandescent light. To properly evaluate alexandrite's colour change, use a true incandescent bulb. A stone that "changes" under fluorescent or LED to a brownish or greyish colour but shows vivid red under incandescent has a stronger change than casual examination under modern artificial light suggests.

Is alexandrite used in Indian Jyotish?

Alexandrite as a Jyotish stone is associated with Rahu (the north lunar node) in some traditions, or is used as a substitute for cat's eye chrysoberyl (Vaidurya) for Ketu in others. Different lineages and practitioners have different views. The critical point for any Jyotish purchase: the stone must be natural (confirmed by major laboratory certificate), and the practitioner must specify whether alexandrite or cat's eye chrysoberyl is required, since they are different stones despite being from the same mineral family. The synthetic risk in the Indian alexandrite market is very high (Behari, 1991; Johari, 1986).

What is "alexandrite effect" in other gems?

Several other gem species show a colour change resembling alexandrite's. Colour-change sapphire (usually blue in daylight, purplish in incandescent) is the most commercially significant. Colour-change garnet (spessartine-pyrope mixes that show green to red or blue to red changes) has produced some striking stones from Tanzania and Madagascar. Colour-change spinel occurs rarely. None of these is alexandrite; they are their own species and should not be sold as alexandrite. The trade phrase "alexandrite effect" or "alexandrite-like colour change" is used for these other species but does not make them alexandrite (GIA Colored Stone identification; Wise, 2016).

Sources cited in this article

  • Nassau, K. (1978). "The Origins of Color in Minerals." American Mineralogist, 63:219–229.
  • Fritsch, E. and Rossman, G.R. (1988). "An Update on Color in Gems, Part 1." Gems and Gemology, 24(2):81–102.
  • Schmetzer, K. (2010). Russian Alexandrites. Schweizerbart Science Publishers, Stuttgart.
  • GIA Gem Reference Guide. (2006). Gemological Institute of America. (pp. 54–57)
  • Wise, R.W. (2016). Secrets of the Gem Trade (2nd ed.). Brunswick House Press. (pp. 141–152)
  • Nassau, K. (1980). Gems Made by Man. Chilton Book Company.
  • GIA Colored Stone identification standards. gia.edu.
  • AGL. Alexandrite identification and colour change grading. aglgemlab.com.
  • Behari, B. (1991). Gems and Astrology. Sagar Publications, New Delhi.
  • GJEPC. Indian alexandrite market data. gjepc.org.