Freboldite is a rare cobalt selenide mineral that typically occurs as massive, metallic-gray aggregates. It is most commonly identified in hydrothermal vein environments associated with other rare selenides and uranium minerals.
Is this freboldite?
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 freboldite with a known reference. Freboldite sits at Mohs <6, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: black.
- 3Read the lusterHold the specimen under a strong light. Luster: metallic.
- 4Match the color rangeCompare against the expected color range: copper red, pink.
- 5Look at form & habitCrystal system: hexagonal. Typical habit: massive, anhedral grains.
Often confused with
Freboldite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Freboldite is noticeably harder (Mohs <6 vs. 2.5-2.75); streak differs, so Freboldite leaves black, Galena leaves lead-gray.
How to tell apart: Freboldite is noticeably harder (Mohs <6 vs. 2.5-3); streak differs, so Freboldite leaves black, Clausthalite leaves grayish black.
How to tell apart: Freboldite is noticeably harder (Mohs <6 vs. 3.5-4.5); streak differs, so Freboldite leaves black, Troilite leaves dark grayish brown.
Often found alongside freboldite
Minerals reported to co-occur with freboldite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CoSe
- Mohs hardness
- <6
- Density
- 7.70 g/cm³
- Colors
- Streak
- Black
- Luster
- Metallic
- Transparency
- Opaque
- Crystal system
- Hexagonal
- Crystal habit
- Massive, anhedral grains
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector, Scientific research
- Host rock
- In dolostone veinlets (Steinbruch, Germany)
Where rockhounds find freboldite
Classic worldwide localities
- Temple Mountain, Emery Co., Utah, USA
- Harz Mountains, from the Trogtal quarry, Steinbruch (type locality), and at Rote Berg, St. Andreasberg, Germany
- Hartenstein, Saxony, Germany
- Pinky Fault uranium deposit, Saskatchewan, Canada
Field-hunting tip
Where it forms: In dolostone veinlets (Steinbruch, Germany). If you start seeing clausthalite, nickeline, guanajuatite in float, you are in the right ground. Typical habit: massive, anhedral grains. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify freboldite?+
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Find freboldite on the map
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