Alum is a group of hydrated double sulfate salts, most commonly referring to potash alum. It often forms as white, crusty, or powdery efflorescences around volcanic vents or in weathered mine dumps, and is notable for its distinct astringent, salty taste.
Is this alum?
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 alum with a known reference. Alum sits at Mohs 2-2.5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Alum leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Alum typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: white, colorless, yellowish, reddish.
- 5Look at form & habitCrystal system: cubic. Typical habit: octahedral, massive, encrusting.
Often confused with
Alum vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside alum
Minerals reported to co-occur with alum. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- KAl(SO₄)₂·12H₂O
- Mohs hardness
- 2-2.5
- Density
- 1.75 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Cubic
- Crystal habit
- Octahedral, Massive, Encrusting
- Cleavage
- None
- Rarity
- Common
- Uses
- Industrial, Collector, Pharmaceutical, Water Treatment
- Host rock
- Volcanic Fumaroles, Oxidized Sulfide Ore Deposits
- Typical price
- $5-30 per specimen
Where rockhounds find alum
2 mapped spotsClassic worldwide localities
- Italy
- Egypt
- USA
- Czech Republic
Field-hunting tip
Look in volcanic fumaroles, oxidized sulfide ore deposits country — that is the host setting where alum typically forms. If you start seeing sulfur, gypsum, pyrite in float, you are in the right ground. Field specimens usually show a octahedral, massive, encrusting habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Massachusetts, Nevada — start trip planning there.
Common questions
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Find alum on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
