Wightmanite is an extremely rare magnesium borate mineral that typically forms as delicate, white, radiating needles or fibrous masses. It is famously associated with the unique boron-rich mineral assemblage found at the Big Creek locality in California, where it occurs within contact-metamorphosed limestone.
Is this wightmanite?
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 wightmanite with a known reference. Wightmanite 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. Wightmanite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Wightmanite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: colorless, white.
- 5Look at form & habitCrystal system: triclinic. Typical habit: acicular or fibrous radial aggregates.
Often confused with
Wightmanite vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside wightmanite
Minerals reported to co-occur with wightmanite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Mg₅(BO₃)O(OH)₂·nH₂O
- Mohs hardness
- 3.5
- Density
- 2.79 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Triclinic
- Crystal habit
- Acicular or Fibrous Radial Aggregates
- Cleavage
- Perfect
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Metamorphosed Limestone
- Typical price
- $50-300 per specimen
Where rockhounds find wightmanite
Classic worldwide localities
- Big Creek, California, USA
Field-hunting tip
Look in metamorphosed limestone country — that is the host setting where wightmanite typically forms. If you start seeing boromuscovite, cahnite, tremolite in float, you are in the right ground. Field specimens usually show a acicular or fibrous radial aggregates habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find wightmanite on the map
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