Cornwallite is a secondary copper arsenate mineral typically forming vibrant green, botryoidal or crusty coatings on matrix. It is highly sought after by collectors for its resemblance to malachite, though it can be distinguished by its distinct habit and associated rare copper minerals. It is predominantly found in the weathered zones of copper-arsenic ore deposits.
Is this cornwallite?
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 cornwallite with a known reference. Cornwallite sits at Mohs 3.5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Cornwallite leaves a pale green streak.
- 3Read the lusterHold the specimen under a strong light. Cornwallite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: green, emerald green, dark green, blackish green.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: botryoidal, crusts, radial aggregates.
Often confused with
Cornwallite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Streak differs — Cornwallite leaves pale green, Malachite leaves light green.
How to tell apart: Streak differs — Cornwallite leaves pale green, Pseudomalachite leaves light green.
How to tell apart: Conichalcite is the harder of the two (Mohs 4.5 vs. 3.5); streak differs — Cornwallite leaves pale green, Conichalcite leaves light green.
Often found alongside cornwallite
Minerals reported to co-occur with cornwallite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Cu₅(AsO₄)₂(OH)₄
- Mohs hardness
- 3.5
- Density
- 4.2-4.3 g/cm³
- Streak
- Pale Green
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Botryoidal, Crusts, Radial Aggregates
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Oxidized Hydrothermal Copper-arsenic Deposits
- Typical price
- $20-150 thumbnail specimen
Where rockhounds find cornwallite
Classic worldwide localities
- Wheal Gorland, Cornwall, England
- Tsumeb, Namibia
- Schwarzwald, Germany
- Laurion, Greece
Field-hunting tip
Look in oxidized hydrothermal copper-arsenic deposits country — that is the host setting where cornwallite typically forms. If you start seeing olivenite, clinoclase, liroconite in float, you are in the right ground. Field specimens usually show a botryoidal, crusts, radial aggregates habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find cornwallite on the map
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