Geminite is a rare copper-arsenic mineral that typically forms delicate, grass-green sprays or radiating acicular crystals. It is primarily found in the oxidation zones of ore deposits where it is often associated with other copper-arsenic secondary minerals. Collectors prize it for its vibrant green color and delicate crystal habit, though it is often small and fragile.
Is this geminite?
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 geminite with a known reference. Geminite sits at Mohs 3-3.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: very pale green.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: light green, sea green.
- 5Look at form & habitCrystal system: triclinic. Typical habit: columnar, radial, twinning common.
Often confused with
Geminite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Geminite leaves very pale green, Malachite leaves pale green; luster reads vitreous on Geminite and adamantine to vitreous; silky if fibrous; dull to earthy if massive on Malachite.
How to tell apart: Conichalcite is the harder of the two (Mohs 4.5 vs. 3-3.5); streak differs, so Geminite leaves very pale green, Conichalcite leaves green.
Often found alongside geminite
Minerals reported to co-occur with geminite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Cu(AsO₃OH)·H₂O
- Mohs hardness
- 3-3.5
- Density
- 3.70 g/cm³
- Colors
- Light greenSea green
- Streak
- Very pale green
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Triclinic
- Crystal habit
- Columnar, radial, twinning common
- Cleavage
- {001}, perfect
- Rarity
- Rare
- Uses
- Collector
- Host rock
- From an arsenic-bearing oxidized Pb–Cu deposit (Cap Garonne mine, France); on specimens from mine dumps at a gold-bearing arsenic sulfide deposit (Salsigne mine, France)
Where rockhounds find geminite
Classic worldwide localities
- Cap Garonne mine, near le Pradet, Var, and in the Salsigne mine, 15 km north of Carcassone, Aude, France
- Jáchymov (Joachimsthal), Czech Republic
- Hartenstein, Saxony, Germany
Field-hunting tip
Where it forms: From an arsenic-bearing oxidized Pb–Cu deposit (Cap Garonne mine, France); on specimens from mine dumps at a gold-bearing arsenic sulfide deposit (Salsigne mine, France). If you start seeing tennantite, covellite, chalcanthite in float, you are in the right ground. Typical habit: columnar, radial, twinning common. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify geminite?+
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Is geminite safe to handle?+
What rocks look like geminite?+
What minerals are found with geminite?+
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Find geminite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
