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Corundum
Encyclopedia

Oxides · Sourced PROFILE

Corundum

Al₂O₃

An aluminium oxide mineral whose gem varieties include ruby and sapphire.

STONES

Related stones

Ruby

The red gem variety of corundum, coloured principally by chromium.

Red to purplish redSource

Sapphire

Gem corundum other than ruby; blue is the best-known colour, while other colours are called fancy sapphire.

Blue and every non-red gem colourSource

AT A GLANCE

Formula
Al₂O₃
Crystal system
Trigonal
Space group
R̅3c
Mohs hardness
9
Density
3.98–4.10 g/cm³
Optical character
Uniaxial (−)

CRYSTALLOGRAPHY

Crystal form & structure

Barrel-shaped, pyramidal, tabular, or prismatic crystals; also granular and massive. Parting and growth features may be prominent.

HAND-SAMPLE PROPERTIES

Physical properties

Cleavage
No true cleavage; common basal and rhombohedral parting
Fracture
Uneven to conchoidal
Tenacity
Brittle, but compact material is tough
Streak
White
Luster
Adamantine to vitreous
Transparency
Transparent to opaque
Observed colours
Colourless, grey, brown, pink, red, orange, yellow, green, blue, violet, or black

LIGHT & OPTICS

Optical properties

Optical class
Uniaxial (−)
Refractive index
nω about 1.767–1.772; nε about 1.759–1.763
Fluorescence & electrical behaviour
Variable; ruby may fluoresce red under ultraviolet light and some material phosphoresces.

GEOLOGICAL SETTING

Formation

Forms in aluminium-rich, silica-poor igneous and metamorphic settings.

MINERAL COMPANY

Common associations

Spinel, magnetite, hematite, calcite, mica, feldspar, and metamorphic or silica-poor igneous minerals

FIELD NOTES

How to identify it

Its hardness of 9 is decisive: it scratches nearly every common mineral. High density and parting help distinguish it; ruby and sapphire are colour varieties of corundum, not separate mineral species.

NAME & HISTORY

Where the name comes from

From Tamil kurundam, a term historically applied to very hard stones.

SCIENTIFIC SOURCE

Handbook of Mineralogy — Corundum

External reference; factual profile paraphrased and attributed. Copyright remains with the publisher.

Open reference PDF

PROVENANCE

Documented occurrences

Pamir orogen

Snezhnoe ruby and sapphire deposit, Kukurt district, Eastern Pamir · Snezhnoe is a marble-hosted ruby and sapphire deposit in the broader Kukurt/Turakulominsky gem-bearing district. The Kukurt name is also used for nearby pegmatite occurrences, so this record keeps the ruby deposit and pegmatite field distinct. Public precision remains regional.

Sourced · Region precisionRubies and Sapphires from Snezhnoe, Tajikistan (Sorokina et al., 2015)