Corundum is an extremely hard aluminum oxide mineral known for its distinctive barrel-shaped hexagonal crystals. While industrial-grade corundum is relatively common, gem-quality varieties like Ruby and Sapphire are highly sought after by collectors and jewelers.
Is this corundum?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch corundum with a known reference. Corundum sits at Mohs 9 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Corundum leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Corundum typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: colorless, red, blue, yellow, green, violet, brown.
- 5Look at form & habitCrystal system: trigonal. Typical habit: hexagonal barrel-shaped crystals, dipyramidal, massive.
Often confused with
Corundum vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside corundum
Minerals reported to co-occur with corundum. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Al₂O₃
- Mohs hardness
- 9
- Density
- 3.98-4.10 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Trigonal
- Crystal habit
- Hexagonal Barrel-shaped Crystals, Dipyramidal, Massive
- Cleavage
- None
- Fluorescence
- Red Varieties Often Fluoresce Red Under LW UV
- Rarity
- Common
- Uses
- Gemstone, Lapidary, Industrial Abrasive
- Host rock
- Metamorphic Rocks Like Marble or Gneiss, And Igneous Pegmatites
- Typical price
- $10-100 for industrial crystals, $500-10000+ for high-quality gemstones
Where rockhounds find corundum
48 mapped spotsClassic worldwide localities
- Ratnapura, Sri Lanka
- Mogok, Myanmar
- Ilakaka, Madagascar
- Kashmir, India
- Montana, USA
U.S. states with corundum
Each link opens a state-specific list of mapped rockhounding spots that produce corundum.
Field-hunting tip
Look in metamorphic rocks like marble or gneiss, and igneous pegmatites country — that is the host setting where corundum typically forms. If you start seeing feldspar, garnet, mica in float, you are in the right ground. Field specimens usually show a hexagonal barrel-shaped crystals, dipyramidal, massive habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in North Carolina, Georgia, Idaho — start trip planning there.
Common questions
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Find corundum on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
