Grimaldiite is an extremely rare chromium oxyhydroxide mineral that typically forms small, shiny black platy crystals. It is most frequently found in serpentinite environments and is highly sought after by advanced mineral collectors due to its scarcity and distinct trigonal symmetry.
Is this grimaldiite?
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 grimaldiite with a known reference. Grimaldiite 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. Grimaldiite leaves a dark brown streak.
- 3Read the lusterHold the specimen under a strong light. Grimaldiite typically shows a metallic luster.
- 4Match the color rangeCompare against the expected color range: black, dark brown.
- 5Look at form & habitCrystal system: trigonal. Typical habit: platy crystals, pseudohexagonal.
Often confused with
Grimaldiite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Iron Ore is the harder of the two (Mohs 5-6.5 vs. 3.5); streak differs — Grimaldiite leaves dark brown, Iron Ore leaves reddish-brown to black; luster reads metallic on Grimaldiite and metallic to submetallic on Iron Ore.
How to tell apart: Lepidocrocite is the harder of the two (Mohs 5 vs. 3.5); streak differs — Grimaldiite leaves dark brown, Lepidocrocite leaves orange-red; luster reads metallic on Grimaldiite and submetallic on Lepidocrocite.
Often found alongside grimaldiite
Minerals reported to co-occur with grimaldiite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CrOOH
- Mohs hardness
- 3.5
- Density
- 4.71 g/cm³
- Colors
- Streak
- Dark Brown
- Luster
- Metallic
- Transparency
- Opaque
- Crystal system
- Trigonal
- Crystal habit
- Platy Crystals, Pseudohexagonal
- Cleavage
- Perfect On {0001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Serpentinite
- Typical price
- $50-300 per specimen
Where rockhounds find grimaldiite
Classic worldwide localities
- Kola Peninsula, Russia
- Sierra Leone
- South Africa
Field-hunting tip
Look in serpentinite country — that is the host setting where grimaldiite typically forms. If you start seeing chromite, uvarovite, clinochlore in float, you are in the right ground. Field specimens usually show a platy crystals, pseudohexagonal habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find grimaldiite on the map
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