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Variety · Oxides & Hydroxide · Simple oxide (hematite group) · Corundum Group · of Corundum

Ruby

Al₂O₃

Corundum turned red by chromium — the hardest colored gem on Earth and the July birthstone.

Hardness9
Crystal systemTrigonal
LusterVitreous
Sp. gravity3.95–4.1
ClassOxides & Hydroxide
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Ruby specimen

Ruby crystals in white marble — the classic Mogok habit — Photo: Parent Géry · CC BY-SA 3.0

What is Ruby?

Ruby is the red variety of corundum — aluminium oxide, Al₂O₃, the very same mineral as sapphire. Swap a little chromium in for aluminium and the crystal turns from colorless to the most celebrated red in the gem world.

Only red corundum earns the name; pinker stones are sold as pink sapphire, and where ‘pink’ ends and ‘ruby’ begins is a famous gem-lab argument. The finest color — a pure, slightly bluish red called pigeon’s blood — comes from the marble mines around Mogok, Myanmar.

Ruby is the July birthstone and, carat for carat, top untreated ruby is among the most expensive gems on Earth.

In one sentence: Ruby is Al₂O₃ plus chromium: hardness 9 — second only to diamond — a glassy luster, and a red glow under ultraviolet light that gave us the world’s first laser.

Physical Properties

The full data table is in the panel alongside. Here’s what those numbers actually mean.

Color and fluorescence. Cr³⁺ replacing Al³⁺ produces the red — and makes ruby fluoresce crimson under UV and even in daylight. That inner glow is why fine rubies seem lit from within.

Hardness. At 9 on the Mohs scale, the corundum family is the hardest naturally occurring mineral after diamond — a ruby ring shrugs off scratches that would scar other gems.

No true cleavage. Corundum has no cleavage, but repeated atomic layers (lamellar twinning) let it part along {0001} and {1011} — a useful clue on rough.

Optics. Uniaxial negative, RI ~1.76–1.77, with strong pleochroism — orangy-red one direction, purplish-red the other. Dense rutile ‘silk’ inside a crystal can produce a six-rayed star: the star ruby.

How & Where It Forms

Ruby forms where aluminium-rich rock is squeezed and heated with very little silica — conditions that favor Al₂O₃ over silicate minerals. The classic host is marble: limestone recrystallized during mountain-building, as at Mogok, Luc Yen and Jegdalek.

A second style is delivered from depth inside volcanic basalt (Thailand, Australia). Rivers then concentrate the durable crystals into gem gravels — the workings of Sri Lanka and Myanmar — where ruby turns up alongside spinel and sapphire.

Often grows alongside:

CalciteSpinelZirconPyritePhlogopite

What It’s Used For

Fine gemstonesJuly birthstone and one of the four precious gems; Burmese pigeon’s-blood stones hold auction records.
LasersTheodore Maiman’s first working laser (1960) used a synthetic ruby rod — chromium makes ruby lase.
Jewel bearingsSynthetic corundum ‘jewels’ give mechanical watches and instruments nearly friction-free pivots.
AbrasivesCorundum’s hardness grinds metal and stone; emery is impure corundum rock.

How to Identify It

Hardness 9 and strong red fluorescence separate ruby from most red gems. Red spinel — history’s great impostor, for the ‘Black Prince’s Ruby’ in the British crown is spinel — is singly refractive, slightly softer, and glows more weakly. Red garnet shows neither pleochroism nor fluorescence. Under a loupe, fine rutile silk or twin-plane parting confirms corundum.

Hardness 9 — scratched only by diamond and moissaniteStrong red fluorescence under UV and even daylightStrong dichroism — orangy vs. purplish redRutile ‘silk’ and twin-plane parting under a loupe
Looks like…But differs because…Quick test
SpinelThe great impostor (the ‘Black Prince’s Ruby’ is spinel) — singly refractive, softer (7.5–8), weaker fluorescencePolariscope
GarnetSingly refractive — no dichroism — and no red UV glowUV light
TourmalineSofter (7–7.5), pyroelectric, different refractive indexRefractometer
ZirconRed ‘hyacinth’ zircon is softer (7.5), far denser, and shows strong facet doublingLoupe & heft

Where It’s Found

Notable localities for fine specimens:

Mogok Stone Tract, Myanmar Mong Hsu, Myanmar Montepuez, Mozambique Ratnapura, Sri Lanka Luc Yen, Vietnam Jegdalek, AfghanistanOpen the locality map →

References & Further Reading

  1. Dana’s New Mineralogy, 8th ed. Corundum group, simple oxides.
  2. Mindat.org — Ruby variety data and locality records.
  3. Wikipedia — Ruby.

Last reviewed 2026-09-14 · How to cite this page.