Colemanite is a significant borate mineral typically found in evaporite deposits in arid regions. It often forms beautiful, glassy, short-prismatic crystals or dense white masses and is highly prized by collectors for its strong white fluorescence under short-wave UV light.
Is this colemanite?
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 colemanite with a known reference. Colemanite sits at Mohs 4.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous to adamantine.
- 4Match the color rangeCompare against the expected color range: colorless, white, gray, yellowish, gray white, yellowish white.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: blocky, crystalline, massive.
Often confused with
Colemanite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Colemanite is noticeably harder (Mohs 4.5 vs. 1.5-2); luster reads vitreous to adamantine on Colemanite and subvitreous, pearly on {010} cleavages, silky if fibrous on Gypsum.
How to tell apart: Colemanite is noticeably harder (Mohs 4.5 vs. 3.5); luster reads vitreous to adamantine on Colemanite and subvitreous, glimmering on Howlite.
How to tell apart: Colemanite is noticeably harder (Mohs 4.5 vs. 2.5).
Often found alongside colemanite
Minerals reported to co-occur with colemanite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca₂B₆O₁₁·5H₂O
- Mohs hardness
- 4.5
- Density
- 2.423 g/cm³
- Streak
- White
- Luster
- Vitreous to adamantine
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
- Crystal habit
- Blocky, crystalline, massive
- Cleavage
- On {010}, perfect; on {001}, distinct
- Fluorescence
- Yellow under SW and LW UV
- Rarity
- Uncommon
- Uses
- Industrial, Collector
- Host rock
- A common constituent in borate deposits formed in arid alkaline lacustrine environments, deficient in sodium and carbonate, typically under warm conditions
Where rockhounds find colemanite
Classic worldwide localities
- Death Valley, near Ryan, Furnace Creek district, in the Thompson mine and the Boraxo mine, California, USA
- Borate, about 10 km northeast of Yermo, Calico Hills, San Bernardino Co., California, USA
- Boron, Kern Co., California, USA
- Anniversary mine, Muddy Mountains district, Clark Co., Nevada, USA
- Penobsquis and Salt Springs evaporite deposits, near Sussex, New Brunswick, Canada
- El Torreon and La Tinaja del Oso, near Magdalena, Sonora, Mexico
Field-hunting tip
Where it forms: A common constituent in borate deposits formed in arid alkaline lacustrine environments, deficient in sodium and carbonate, typically under warm conditions. If you start seeing howlite, ulexite, searlesite in float, you are in the right ground. Typical habit: blocky, crystalline, massive. Train your eye for that shape before scanning the outcrop.
Common questions
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Find colemanite on the map
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