Bentonite is an absorbent clay formed by the chemical weathering of volcanic ash in water. It is known for its remarkable ability to swell significantly when wetted, making it highly valuable for sealing and industrial binding applications.

Hardness

1-2

Mohs

Luster

Dull

Streak

White

Transparency

Opaque

sedimentarycommonSmectite Group family

Bentonite

Also: Montmorillonite Clay

Hardness: 1-2
Luster: Dull
Streak: White
Transparency: Opaque

Is this bentonite?

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 bentonite with a known reference. Bentonite sits at Mohs 1-2 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Bentonite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Bentonite typically shows a dull luster.
4
Match the color range
Compare against the expected color range: white, cream, gray, yellow, green.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: massive.

Often confused with

Bentonite vs. its common look-alikes — and how to tell them apart in the field.

How to tell apart: Luster reads dull on Bentonite and pearly on Illite.

How to tell apart: Luster reads dull on Bentonite and pearly on Talc.

Compare →

Often found alongside bentonite

Minerals reported to co-occur with bentonite. Spotting these in float or country rock is a strong cue you are in the right ground.

All properties

Mohs hardness: 1-2
Density: 2.0-2.7 g/cm³
Colors: White Cream Gray Yellow Green
Streak: White
Luster: Dull
Transparency: Opaque
Crystal system: Monoclinic
Crystal habit: Massive
Cleavage: Perfect
Rarity: Common
Uses: Industrial, Drilling Fluid, Cat Litter, Binder
Host rock: Altered Volcanic Ash Deposits
Typical price: $5-20 per kilogram

Where rockhounds find bentonite

2 mapped spots

Classic worldwide localities

Wyoming, USA
Montana, USA
Greece
Turkey
Italy

Field-hunting tip

Look in altered volcanic ash deposits country — that is the host setting where bentonite typically forms. If you start seeing quartz, calcite, gypsum in float, you are in the right ground. Field specimens usually show a massive habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in California, Montana — start trip planning there.

Browse 2 U.S. spots for bentonite →

Common questions

How do you identify bentonite?+

Mohs hardness is 1-2. It typically shows a dull luster. The streak is white. Common colors include white, cream, gray, yellow.

Where is bentonite found?+

Notable localities include Wyoming, USA; Montana, USA; Greece; Turkey; Italy.

Can I find bentonite in the United States?+

RockHoundR maps 2 bentonite rockhounding spots across 2 U.S. states — the top states are California, Montana.

How much is bentonite worth?+

Typical asking prices fall in the range of $5-20 per kilogram. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like bentonite?+

Bentonite is most often confused with Kaolinite, Illite, Talc. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with bentonite?+

Bentonite commonly co-occurs with Quartz, Calcite, Gypsum, Feldspar. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does bentonite form in?+

Bentonite typically forms in altered volcanic ash deposits. Working float back to the host body is the standard way to chase a fresh occurrence.

What is bentonite used for?+

Bentonite is used in industrial, drilling fluid, cat litter, binder.

Find bentonite on the map

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