Crystal Structure
Hexagonal close-packed O²⁻ with Al³⁺ in 2/3 of octahedral sites.
Elemental Composition (by mass)
| Element | Mass % | Visual |
|---|---|---|
| Al Aluminum | 52.93% | |
| O Oxygen | 47.07% |
Computed from simplified end-member formula. Solid-solution series, water content, and trace substitutions cause real-world variation.
IMA Abbreviation (Whitney-Evans 2010)
Crn
→ Corundum
Used widely in petrology
Standard symbol from American Mineralogist (Whitney & Evans, 2010). Used in thin-section labeling, phase diagrams, and IMA-style species records.
Pronunciation
/kəˈrʌndəm/
↔ kuh-RUN-dum
three syllables (Sanskrit kuruvinda)
Lapidary & Faceting Recommendations
Recommended cut:
oval / cushion
Also seen:
cabochon (star), cushion
Typical yield:
30% of rough
See ruby / sapphire.
UV Fluorescence
SW (254 nm)
Red (ruby) / none (sapphire)
weak
LW (365 nm)
Red
moderate
Cr-doped material fluoresces red; pure corundum and Fe-doped sapphire inert.
SW = shortwave (germicidal lamp). LW = longwave (blacklight). Response varies with locality, trace impurities, and treatment.
Tenacity
Behavior:
brittle to tough
Under stress:
Mohs 9 — chips under hammer
Hard and somewhat brittle; preserved by parting rather than cleavage.
Luster
vitreous→adamantine
High-quality ruby/sapphire borders on adamantine.
Diaphaneity (Transparency)
transparent-to-translucent
Gem-grade ruby/sapphire transparent; massive corundum opaque.
Type Locality
(Sanskrit "kuruvinda") — Tamil Nadu, India
Described 1801 by Greville
Magnetism
Category:
diamagnetic
Test result:
Slight repulsion
Al₂O₃ pure form; ruby (Cr-bearing) slightly paramagnetic.
Test with rare-earth magnet (N42 or N52 neodymium). Suspend specimen on thread for sensitive paramagnetic detection. Diamagnetic minerals are weakly repelled (visible only with strong magnets like bismuth).
Diagnostic Field Tests
Hardness→ Mohs 9 — scratches everything except diamond/moissanite
Scratches topaz (Mohs 8).
SG check→ ~4.0 — heavier than expected for clear material
Useful for separation from glass.
⚠ Use dilute HCl (~10%) only on inconspicuous spots; rinse promptly. Smell-tests should be brief and ventilated. Taste-test ONLY halite/sylvite — never lead, arsenic, or sulfur minerals.
Specific Gravity
3.97–4.05
g/cm³
heavy
Including ruby and sapphire — heavy for a "colorless" appearance.
For comparison: water = 1.00, glass ≈ 2.5, quartz = 2.65, corundum ≈ 4.00, galena ≈ 7.50, gold ≈ 19.3.
Geological Setting
Burma marble-hosted ruby is metamorphic. Basalt-hosted (Thailand, Australia) is alkali-basaltic xenocryst origin.
Characteristic Inclusions
Silksolid★ diagnostic
Generic for all corundum — fine rutile needles at 120°.
★ Diagnostic inclusions are characteristic enough to help identify origin or species under 10× loupe.
Twinning Laws
Rhombohedral partingparting
Polysynthetic twinning on {1011} weakens crystal — produces parting that mimics cleavage.
Formation eraHigh-T metamorphic; Burmese sapphires Cenozoic alkaline basalt.
Cleavage & Fracture
Cleavage:
none — (parting on {0001}/{1011})
Fracture:
conchoidal / uneven
No true cleavage but parting along basal and rhombohedral planes.
Collector tier: Cabinet Classic
World-class display species — sought after for cabinet collections, well-documented localities, frequent show-piece pieces.
Named varieties — color spectrum
Mohs 9
Vickers (~) 2300 HV
Knoop (~) 1800 HK
Nickel–Strunz
4.CB.05Dana
04.03.01.01Geological setting
Metamorphic
Diagnostic properties
Hardness 9
Element composition by mass
Formula: Al₂O₃ · molar mass: 101.96 g/mol
| Al | 52.93% | |
| O | 47.07% |
Computed from atomic weights (IUPAC 2021). Site-occupancy groups (Fe,Mn) split equally.
GroupCorundum / Sapphire Family
Optical Effects
Asterism
Mohs Hardness
9
Corundum sits at 9 on the Mohs scale — one of the hardest natural materials known.
Colors:
Streak
White
White
Crystal system
Trigonal
Trigonal
Pronunciation/kəˈrʌndəm/
Discovery First described 1798 by Robert Jameson (India)
Oxides & HydroxidesOxides
TL;DR · 1 min read
Corundum (Al₂O₃) is the parent species of two of the most prized gem minerals on Earth — ruby (red, chromium-colored) and sapphire (every other color: blue, pink, yellow, green, padparadscha). It is the second-hardest natural mineral after diamond, with a Mohs hardness of 9, and crystallizes in the trigonal system as hexagonal prisms, barrels, and tabular crystals.
Corundum (Al₂O₃) is the parent species of two of the most prized gem minerals on Earth — ruby (red, chromium-colored) and sapphire (every other color: blue, pink, yellow, green, padparadscha). It is the second-hardest natural mineral after diamond, with a Mohs hardness of 9, and crystallizes in the trigonal system as hexagonal prisms, barrels, and tabular crystals. Mogok (Myanmar) and Sri Lanka are the historical world standards; Yunnan hosts modest Chinese deposits.
More minerals to explore
Pyrolusite (软锰矿)
Tetragonal
Anatase (锐钛矿)
Tetragonal
Brookite (板钛矿)
Orthorhombic
Rutile (金红石)
Tetragonal
Magnetite (磁铁矿)
Isometric (Cubic)
Zincite (红锌矿)
Hexagonal
Varieties of Corundum
Corundum is a parent species — the following named varieties differ in color or chemistry but share the same fundamental mineralogy.
- Ruby (u7ea2u5b9du77f3)redChromium-bearing red corundum variety; one of the four cardinal gems.
- Sapphire (u84ddu5b9du77f3)blue and other non-red colorsIron- and titanium-bearing corundum, traditionally blue but also yellow, pink, green, etc.
- Star Sapphire (u661fu5149u84ddu5b9du77f3)asterismSapphire showing six-rayed star asterism due to oriented rutile inclusions.
- Padparadscha (u5e15u5e15u62c9u6070)pink-orangeRare pink-orange sapphire variety; named after lotus blossom in Sinhalese.