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

Sapphire

Al₂O₃

Every color of corundum that isn’t red — royal blue from iron and titanium, plus a whole ‘fancy’ rainbow. September’s birthstone.

Hardness9
Crystal systemTrigonal
LusterVitreous
Sp. gravity3.95–4.1
ClassOxides & Hydroxide
←PrevSapozhnikoviteNextSapphirine→
Sapphire specimen

Blue sapphire crystal — corundum colored by iron–titanium charge transfer — Photo: Deerstop · CC Attribution 2.0 Generic

What is Sapphire?

Sapphire is corundum in every color except red — red belongs to ruby. Same mineral, same formula Al₂O₃, same hardness 9; only the trace elements differ. Blue sapphire, the classic, owes its color to an electron-passing partnership between iron and titanium atoms.

Non-blue stones are ‘fancy sapphires’: yellow, pink, green, purple, orange — and the prized pink-orange padparadscha, named for the lotus blossom. Some change color between daylight and lamplight; dense needles of rutile give others a six-rayed star.

Sapphire is September’s birthstone and arguably the world’s most-worn colored gem — engagement rings have favored it since antiquity.

In one sentence: Sapphire is Al₂O₃ in a thousand disguises: hardness 9, glassy luster, no cleavage — and a blue caused by electrons shuttling between iron and titanium atoms.

Physical Properties

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

Color. In blue sapphire, Fe²⁺ and Ti⁴⁺ ions sit side by side and pass an electron back and forth — an intervalence charge transfer that swallows red light and returns blue. Fe³⁺ alone gives yellow; chromium gives pink; mixtures paint the fancy spectrum.

Hardness & toughness. 9 on Mohs with no cleavage makes sapphire one of the toughest gems — suitable for watch faces and phone lenses as well as rings.

Optics. Uniaxial negative, RI ~1.76–1.77, with strong dichroism — deep blue one direction, greenish the other. Fine rutile needles produce silk; when dense, a six-rayed star sapphire.

Treatment. Most sapphires are heat-treated to deepen or clarify color — an accepted practice that gem labs routinely disclose.

How & Where It Forms

Sapphire grows wherever aluminium is abundant and silica scarce: in metamorphosed shales and gneisses, in marble, and around silica-poor pegmatitic rocks. The fabled Kashmir stones formed where pegmatitic fluids met impure limestone high in the Himalaya.

A second route is volcanic: sapphire crystallizes deep in the Earth and rides to the surface inside alkali basalt — the source of the vast Australian, Thai and Montana fields. Rivers do the final sorting, delivering the gem gravels of Ratnapura and Ilakaka.

Often grows alongside:

ZirconSpinelGarnetKyaniteOlivine

What It’s Used For

Fine gemstonesSeptember birthstone and the benchmark blue gem — a favorite for engagement rings since antiquity.
Scratch-proof surfacesSynthetic sapphire crystal covers watch faces, camera lenses and scanner windows.
Electronics & lasersSapphire wafers carry the gallium nitride of white LEDs; Ti:sapphire crystals drive tunable research lasers.
Jewel bearingsSynthetic corundum jewels keep watch movements and precision gauges running friction-free.

How to Identify It

Hardness 9 and strong dichroism do most of the work. Blue spinel is singly refractive and lighter; tanzanite is softer and shows three pleochroic colors including violet; blue zircon flashes dramatic facet doubling and is far denser. Iolite reveals a near-colorless axis, and benitoite out-fires diamond and glows blue under short-wave UV.

Hardness 9 — scratched only by diamond and moissaniteStrong dichroism — deep blue vs. greenishRutile ‘silk’; dense silk gives six-rayed star stonesNo cleavage; stepped parting on basal surfaces
Looks like…But differs because…Quick test
SpinelSingly refractive and lighter (SG ~3.6) — a classic blue simulantPolariscope
TanzaniteSofter (6–7) and trichroic — violet flashes sapphire never showsDichroscope
ZirconFar denser (~4.7) with dramatic facet doubling and adamantine lusterLoupe & heft
IoliteTrichroic with a near-colorless axis; softer (7–7.5)Pleochroism
BenitoiteDispersion stronger than diamond’s; fluoresces bright blue under short-wave UVUV & fire

Where It’s Found

Notable localities for fine specimens:

Padder Valley, Kashmir, India Ratnapura, Sri Lanka Ilakaka, Madagascar Yogo Gulch, USA Bo Phloi, Kanchanaburi, Thailand Anakie–Rubyvale, AustraliaOpen the locality map →

References & Further Reading

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

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