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.

Hardness

3.5

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent

mineralrare

Wightmanite

Hardness: 3.5
Luster: Vitreous
Streak: White
Transparency: Transparent

Is this wightmanite?

5-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try to scratch wightmanite with a known reference. Wightmanite sits at Mohs 3.5, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Wightmanite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Wightmanite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: colorless, white.
5
Look at form & habit
Crystal 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.

mineral

Boromuscovite

Mohs 2.5-3

How to tell apart: Luster reads vitreous on Wightmanite and pearly on Boromuscovite.

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
Colors: Colorless White
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, 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

How do you identify wightmanite?+

Mohs hardness is 3.5. It typically shows a vitreous luster. The streak is white. Common colors include colorless, white.

Where is wightmanite found?+

Notable localities include Big Creek, California, USA.

How much is wightmanite worth?+

Typical asking prices fall in the range of $50-300 per specimen. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like wightmanite?+

Wightmanite is most often confused with Boromuscovite, Cahnite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with wightmanite?+

Wightmanite commonly co-occurs with boromuscovite, cahnite, tremolite, calcite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does wightmanite form in?+

Wightmanite typically forms in metamorphosed limestone. Working float back to the host body is the standard way to chase a fresh occurrence.

What is wightmanite used for?+

Wightmanite is used in collector.

Find wightmanite on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.