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, so 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, so 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, so 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, 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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