Cronstedtite is a rare member of the serpentine group often found in hydrothermal veins as striking triangular plates or pseudo-hexagonal pyramids. Collectors prize it for its unique metallic to vitreous luster and distinct geometry, though it is usually found only in small, delicate specimens.
Is this cronstedtite?
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 cronstedtite with a known reference. Cronstedtite 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. Cronstedtite leaves a dark greenish-gray streak.
- 3Read the lusterHold the specimen under a strong light. Cronstedtite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: black, brownish-black, dark green.
- 5Look at form & habitCrystal system: trigonal. Typical habit: hexagonal pyramidal crystals, triangular plates, or radial aggregates.
Often confused with
Cronstedtite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Streak differs — Cronstedtite leaves dark greenish-gray, Chamosite leaves white to pale green; luster reads vitreous on Cronstedtite and pearly on Chamosite.
How to tell apart: Streak differs — Cronstedtite leaves dark greenish-gray, Lizardite leaves white; luster reads vitreous on Cronstedtite and greasy on Lizardite.
How to tell apart: Cronstedtite is noticeably harder (Mohs 3.5 vs. 2-2.5); streak differs — Cronstedtite leaves dark greenish-gray, Clinochlore leaves white; luster reads vitreous on Cronstedtite and pearly on Clinochlore.
Often found alongside cronstedtite
Minerals reported to co-occur with cronstedtite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Fe²⁺₂Fe³⁺(Si,Fe³⁺)₂O₅(OH)₄
- Mohs hardness
- 3.5
- Density
- 3.34-3.37 g/cm³
- Streak
- Dark Greenish-gray
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Trigonal
- Crystal habit
- Hexagonal Pyramidal Crystals, Triangular Plates, Or Radial Aggregates
- Cleavage
- Perfect Basal
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Hydrothermal Veins
- Typical price
- $20-150 thumbnail specimens
Where rockhounds find cronstedtite
Classic worldwide localities
- Pribram, Czech Republic
- Cornwall, England
- Siegen, Germany
- Kongsberg, Norway
Field-hunting tip
Look in hydrothermal veins country — that is the host setting where cronstedtite typically forms. If you start seeing siderite, pyrite, sphalerite in float, you are in the right ground. Field specimens usually show a hexagonal pyramidal crystals, triangular plates, or radial aggregates habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find cronstedtite on the map
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