Corundum
Corundum is aluminium oxide, Al2O3: hardness 9, density 3.98 to 4.10, no cleavage. Ruby and sapphire are its varieties. How to tell it from red garnet, worked.
Corundum is aluminium oxide, Al2O3: hexagonal, hardness 9, measured density 3.98 to 4.10, no cleavage but partings, adamantine to vitreous lustre. Pure corundum is colourless; GIA attributes ruby's red to chromium and blue sapphire to iron and titanium. Ruby and sapphire are varieties of this one species, not separate IMA minerals.
In short
- One species, many gem names. The September 2026 IMA-CNMNC Master List lists corundum as Al2O3, grandfathered, with the year 1714 and India each queried. GIA calls ruby and sapphire varieties of corundum, and says the name sapphire can apply to any corundum that is not red. A script over the IMA list finds no row for ruby, sapphire or padparadscha.
- The colour is a trace element. GIA: pure corundum is colourless; chromium causes ruby’s red and the pink of pink sapphire; iron and titanium make blue sapphire, where only a few hundredths of a percent can cause the colour. The Handbook of Mineralogy sheet’s chemistry line names the same three, Fe, Ti and Cr, as traces in nearly pure Al2O3, which is 52.93% aluminium by mass, worked.
- Ruby or red garnet? The one-reading rule. Almandine, spessartine and grossular are cubic: each sheet prints a single index, 1.830, 1.800 and 1.734. Corundum is uniaxial, with two, ω 1.767 to 1.772 and ε 1.759 to 1.763. A red stone that gives one index is behaving as a garnet; the nearest garnet value is 0.025 from corundum’s range.
- Documented failure: the scale. A 20.00 g piece reads 14.97 to 15.12 g in water as corundum. Grossular, 14.44 g, is lighter; spessartine, 15.23 g, and almandine, 15.37 g, are heavier. The garnets bracket corundum, and spessartine is 0.11 g away, so weighing alone does not name a red stone.
- Ruby fluoresces red, sometimes. The sheet says corundum may fluoresce or phosphoresce under UV. GIA says chromium can cause red fluorescence, that marble-hosted rubies typically fluoresce red even in sunlight, and that higher iron can mask it. GIA also notes the first laser, in 1960, used ruby’s red fluorescence.
| Species | IMA formula | Crystal system; optical class on the sheet | Refractive indices on the sheet | Indices printed | Hardness | Density (measured) | 20.00 g piece in water |
|---|---|---|---|---|---|---|---|
| Corundum (ruby, sapphire) | Al2O3 | Hexagonal; uniaxial (–), commonly anomalously biaxial | ω 1.767 to 1.772; ε 1.759 to 1.763 | Two | 9 | 3.98 to 4.10 | 14.97 to 15.12 g |
| Almandine (garnet) | Fe2+3Al2(SiO4)3 | Cubic; isotropic, anomalously biaxial | n = 1.830 | One | 7 to 7.5 | 4.318 | 15.37 g |
| Spessartine (garnet) | Mn2+3Al2(SiO4)3 | Cubic; isotropic, may show weak anisotropism | n = 1.800 | One | 7 to 7.5 | 4.190 | 15.23 g |
| Grossular (garnet) | Ca3Al2(SiO4)3 | Cubic; isotropic, weak strain birefringence | n = 1.734 | One | 6.5 to 7 | 3.594 | 14.44 g |
| Elbaite (tourmaline) | Na(Al1.5Li1.5)Al6(Si6O18)(BO3)3(OH)3(OH) | Hexagonal; uniaxial (–) | ω 1.633 to 1.651; ε 1.615 to 1.630 | Two | 7 | 2.90 to 3.10 | 13.10 to 13.55 g |
| Chrysoberyl | BeAl2O4 | Orthorhombic; biaxial (+) | α 1.746; β 1.748; γ 1.756 | Three | 8.5 | 3.75 | 14.67 g |
What is corundum? Aluminium oxide of the hematite group
The names. The IMA-CNMNC Master List writes Al2O3, status G, with the year of first description given as 1714 and the country as India, each followed by a question mark. The Handbook sheet writes the same formula and places corundum in the Hematite group. The name is probably from the Sanskrit kurivinda, for ruby, through the Tamil kurundam, in the sheet’s words. A script over the IMA list finds one row with the formula Al2O3: corundum.
What it looks like. Hexagonal, point group -3 2/m. Crystals are prismatic or steeply dipyramidal, tabular or rhombohedral, rarely acicular, typically rough, to 1 m, sectorially striated on {0001} parallel to {10-11}. There is no cleavage: the sheet records partings on {0001} and {10-11} from exsolved böhmite, and common lamellar twinning parallel to {10-11}. Hardness is 9, the lustre adamantine to vitreous, the streak white. The vocabulary is on cleavage or fracture.
Where it forms. The sheet calls corundum characteristic of aluminium-rich, silicon-poor environments: syenite and monzonite, some quartz-free pegmatites, eclogitic xenoliths in kimberlites, high-grade metamorphic aluminous rocks and metamorphosed limestones, and detrital in placers. GIA makes the same point: corundum needs a growth environment free of silicon, which is why natural corundum is relatively uncommon.
Is ruby corundum? Ruby, sapphire and the other variety names
Ruby is red corundum. GIA states that chromium is the trace element that causes ruby’s red, which ranges from orangy red to purplish red, and that more chromium gives a stronger red. GIA’s facts panel gives refractive index 1.762 to 1.770, birefringence 0.008 to 0.010, specific gravity 4.00 (±0.05) and hardness 9; the specific gravity sits inside the sheet’s 3.98 to 4.10.
Sapphire is, per GIA, any corundum that is not red: blue when the trace elements are iron and titanium, darker with more iron, and violet, green, yellow, orange, pink and purple as fancy sapphires. GIA also names padparadscha, a pinkish-orange sapphire; star corundum, where needle-like oriented inclusions give a six-ray star on a cabochon (the sheet’s word is asteriated); and colour-change sapphire, most often blue in daylight and purple in incandescent light. That last is not alexandrite, which is a different species: see is chrysoberyl the same as alexandrite.
Treatment and synthetics. GIA notes that processes exist to alter colour, clarity and durability, and that synthetic sapphire is durable enough for supermarket scanner windows. This page describes no treatment and makes no claim about any stone; how to tell if a crystal is fake covers the general checks.
How to tell ruby from garnet: the one-reading rule
The rule. A refractive index reading that shows one value points to a garnet; two values point to corundum. The reason is crystal system. The almandine, spessartine and grossular sheets are cubic and isotropic, each printing a single index: 1.830, 1.800 and 1.734. Corundum is uniaxial and prints two, ω 1.767 to 1.772 and ε 1.759 to 1.763. The nearest garnet, grossular, is 0.025 below corundum’s lowest value and spessartine 0.028 above its highest.
Where the rule needs care. The sheets record that almandine is anomalously biaxial, spessartine may show weak anisotropism, grossular weak strain birefringence, and corundum commonly anomalously biaxial, so a polariscope alone can mislead in both directions. Red elbaite also prints two indices, but they are far lower, 1.615 to 1.651. Chrysoberyl prints three; its own species page sets out its two-bracket rule against topaz and corundum, which is not repeated here.
Documented failure: the scale. A clean 20.00 g corundum reads 14.97 to 15.12 g in water. Grossular reads 14.44 g; spessartine 15.23 g and almandine 15.37 g. The garnets bracket corundum, spessartine only 0.11 g above it, so weight can rule out elbaite (13.10 to 13.55 g) but cannot name a red stone as corundum. Hardness does separate them, 9 against 6.5 to 7.5, but a scratch test damages a cut stone; how to measure specific gravity at home and how to test mineral hardness properly give the methods. The garnet family is compared on is rhodonite a garnet.
Where corundum is found: Mogok, Sri Lanka and Franklin
The Distribution field opens numerous localities. It names red gems from the Mogok district, Myanmar; the Ratnapura district, Sri Lanka; around Mysore, India; the Jegdalek marble near Sorobi, Afghanistan; Merkestein near Longido and the Morogoro district, Tanzania; and Ampanihy, Madagascar. Blue, green and yellow gems come, per the sheet, from Chanthaburi and Trat, Thailand; Pailin, Cambodia; the Zanskar district, Kashmir; the Umba Valley, Tanzania; Andranondambo, Madagascar; Anakie, Queensland; and Yogo Gulch, Montana.
Crystal localities on the sheet include Chester, Massachusetts; the Cortland district, New York; Franklin, New Jersey; large crystals from Hogback Mountain and Buck Creek, North Carolina; Bancroft and Haliburton, Ontario; Naxos and Samos, Greece; and large crystals from around the Soutpansberg, South Africa. GIA adds that the most renowned rubies, from Myanmar, the Himalayas and northern Vietnam, typically form in marble. 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.