Chrysoberyl
Chrysoberyl is BeAl2O4, hardness 8.5, density 3.75; alexandrite is its colour-change variety. Why weighing fails on a cut stone and refractive index does not.
Chrysoberyl is beryllium aluminium oxide, BeAl2O4: orthorhombic, hardness 8.5, measured density 3.75, refractive indices 1.746 to 1.756, green, yellow or brownish. Alexandrite is its colour-change variety, green in daylight and red in incandescent light, and chrysoberyl with oriented inclusions gives cat's-eye stones. Neither variety name is an IMA species.
In short
- One species, two famous varieties. The September 2026 IMA-CNMNC Master List lists chrysoberyl, BeAl2O4, grandfathered, 1789, Brazil. GIA calls alexandrite a rare variety of the mineral chrysoberyl that changes from green in daylight to brownish or purplish red in incandescent light. The Handbook of Mineralogy sheet records that chrysoberyl may be opaque and chatoyant with oriented inclusions: the cat’s-eye.
- The two-bracket rule. In the table’s six gem minerals, chrysoberyl’s hardness 8.5 sits between topaz (8) and corundum (9), and so does its weight: a 20.00 g piece reads 14.67 g in water, against 14.27 to 14.40 g for topaz and 14.97 to 15.12 g for corundum. Both neighbours are within 0.31 g.
- Why weighing fails on a cut stone. Scale the same arithmetic to a 2.00 ct stone, 0.400 g: chrysoberyl reads 0.2933 g in water, topaz at most 0.2880 g, corundum at least 0.2995 g. Both gaps are under 0.01 g, below the last digit of a 0.01 g balance.
- Refractive index does not overlap. On the sheets, chrysoberyl’s highest index, 1.756, is below corundum’s lowest, 1.759, and far above topaz’s highest, 1.644. GIA gives alexandrite 1.746 to 1.755. The International Gem Society names synthetic corundum with an alexandrite-like colour change as a simulant.
- Brazil and the Urals. The sheet names Minas Gerais, Espírito Santo and Bahia, the Izumrudnye mines near Yekaterinburg in the Urals, and gem gravels of the Ratnapura district, Sri Lanka. GIA dates alexandrite’s discovery to Russia’s Ural Mountains in the 1830s.
| Species | IMA formula | Hardness | Density (measured) | 20.00 g piece in water | 2.00 ct stone in water (density limit nearest chrysoberyl) | Refractive indices on the sheet | Overlaps chrysoberyl's indices? |
|---|---|---|---|---|---|---|---|
| Chrysoberyl | BeAl2O4 | 8.5 | 3.75 | 14.67 g | 0.2933 g | 1.746 to 1.756 | — |
| Corundum | Al2O3 | 9 | 3.98 to 4.10 | 14.97 to 15.12 g | 0.2995 g | 1.759 to 1.772 | No: 0.003 above |
| Topaz | Al2SiO4F2 | 8 | 3.49 to 3.57 | 14.27 to 14.40 g | 0.2880 g | 1.606 to 1.644 | No |
| Beryl | Be3Al2(Si6O18) | 7.5 to 8 | 2.63 to 2.97 | 12.40 to 13.27 g | 0.2653 g | 1.565 to 1.610 | No |
| Forsterite (peridot) | Mg2(SiO4) | 7 | 3.275 | 13.89 g | 0.2779 g | 1.635 to 1.670 | No |
| Quartz | SiO2 | 7 | 2.65 | 12.45 g | 0.2491 g | 1.544 to 1.553 | No |
What is chrysoberyl? Beryllium aluminium oxide, not a beryl
The name misleads. The sheet derives it from the Greek for golden, for the colour, and beryl. But chrysoberyl is an oxide, BeAl2O4, orthorhombic, while beryl is a ring silicate, Be3Al2(Si6O18), hexagonal. They share beryllium and aluminium and often a pegmatite: the chrysoberyl sheet lists beryl among its granite-pegmatite associates, with quartz, muscovite, albite, columbite, tourmaline, topaz, kyanite and staurolite.
Composition, worked on standard atomic weights: formula mass 126.97; 19.70% BeO and 80.30% Al2O3, the sheet’s ideal column, or 7.10% beryllium, the figure on the live Minas Gerais page. The sheet’s analysis from near Golden, Colorado, has 3.60% FeO and 0.55% TiO2 besides.
Habit. Crystals are tabular or short prismatic, to 22 cm, and prominently striated. Twinning is common and gives the shapes collectors know: flattened heart-shaped or pseudohexagonal multiple contact and penetration twins, in the sheet’s words. Mineral twinning explained covers the vocabulary. Cleavage is distinct on one form, imperfect on a second and poor on a third; fracture uneven to conchoidal.
Is alexandrite chrysoberyl? Yes, and so is cat's-eye
Alexandrite, per GIA, is a rare variety of the mineral chrysoberyl. Its colour is green in daylight or fluorescent light, changing to brownish or purplish red in incandescent light. GIA’s facts panel gives mineral chrysoberyl, BeAl2O4; refractive index 1.746 to 1.755; birefringence 0.008 to 0.010; specific gravity 3.73; Mohs hardness 8.5. GIA states that the colour change does not arise from pleochroism, though alexandrite is strongly pleochroic: green, orange and purple-red.
What the Handbook adds. The sheet’s colour line says chrysoberyl may be raspberry-red under incandescent light when chromian; its pleochroism is X red, Y yellow-orange, Z emerald-green. Alexandrite is not a row on the IMA list. The International Gem Society notes that not every colour-change chrysoberyl is an alexandrite, and that no gemmological consensus restricts the name to a classic range of colour shift. This page draws no line either.
Cat’s-eye. The Handbook records that chrysoberyl may be opaque and chatoyant with oriented inclusions. The International Gem Society calls chrysoberyl well known for a chatoyancy or cat’s-eye effect when cut as cabochons, and GIA notes that alexandrite can show both colour change and a cat’s-eye. This page gives no buying or grading advice.
How to tell chrysoberyl from topaz and corundum: the two-bracket rule
The brackets. On the Handbook sheets, topaz is hardness 8 and corundum 9; chrysoberyl, 8.5, sits between them. Weight brackets it the same way. A 20.00 g piece reads 14.67 g in water as chrysoberyl, 14.27 to 14.40 g as topaz and 14.97 to 15.12 g as corundum: 0.27 g below and 0.31 g above at the nearest. On a clean rough crystal that is 27 to 31 steps on a 0.01 g balance, and the method is on how to measure specific gravity at home.
Documented failure: the carat problem. A carat is 0.200 g, so a 2.00 ct stone is 0.400 g. Its in-water weight is 0.2933 g as chrysoberyl, 0.2880 g at topaz’s densest and 0.2995 g at corundum’s lightest. The gaps are 0.0053 g and 0.0062 g, under the last digit of a 0.01 g balance. Hardness is no help on a cut stone, because a scratch test damages it.
What does separate them: refractive index. The sheets give chrysoberyl 1.746 to 1.756, corundum 1.759 to 1.772 and topaz 1.606 to 1.644. Of the five other minerals in the table, none overlaps chrysoberyl. Corundum is the close one, 0.003 above on the sheets’ figures; whether a given instrument resolves that is not something these sources state. The International Gem Society names synthetic corundum with an alexandrite-like colour change among alexandrite simulants, which is why corundum is in the table. Dyed or fake material in general is on how to tell if a crystal is fake.
Where chrysoberyl is found: Brazil, the Urals and Sri Lanka
Occurrence, per the sheet: characteristic of some granite pegmatites associated with high-grade mica schists, or reaction zones in ultramafic rocks; also detrital in placers. The Distribution field opens widespread, however fine crystals are uncommon.
Named localities. In Brazil: exceptional crystals from Tancredo, Itaguaçu and Colatina, Espírito Santo; Faria Lemos, Santa Luzia de Carangola and Americana, Teófilo Otoni, Minas Gerais; and Campo Formoso, Teixeira de Freitas and Cachoeira, Bahia. Also Maršíkov, Czech Republic; the Izumrudnye mines, Tokovaya River, near Yekaterinburg, and Mursinka, Ural Mountains, Russia; Miakanjovato, Madagascar; near Masvingo, Zimbabwe; gem-gravel placers in the Ratnapura district, Sri Lanka; and near Golden, Colorado, and Topsham, Paris and Hartford, Maine, USA.
Alexandrite’s history, per GIA: discovered in Russia’s Ural Mountains in the 1830s, named for Czar Alexander II, and now also found in Sri Lanka, East Africa and Brazil. No British locality is on the sheet. The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.