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, 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.
