Mudstone is a fine-grained sedimentary rock formed from the compaction of silt and clay particles. Unlike shale, it lacks fissility, meaning it does not split into thin, parallel layers. It is ubiquitous in sedimentary sequences worldwide and often serves as a source rock for hydrocarbons.

Hardness

2-3

Mohs

Luster

Dull

Transparency

Opaque

sedimentarycommon

Mudstone

Hardness: 2-3
Luster: Dull
Transparency: Opaque

Is this mudstone?

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 mudstone with a known reference. Mudstone sits at Mohs 2-3, softer than the next harder reference, harder than the previous one.
2
Read the texture
Look at a fresh break with a hand lens. Texture: Not laminated or fissile. Grain size: Grains under 0.063 mm.
3
Pick out the minerals
Composition: Mostly silicate grains, a mix of silt- and clay-sized particles. Identify the two or three most abundant grains.
4
Read the luster
Hold the specimen under a strong light. Luster: dull.
5
Match the color range
Compare against the expected color range: gray, brown, red, black.

Often confused with

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

Often found alongside mudstone

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

All properties

Mohs hardness: 2-3
Density: 2.0-2.6 g/cm³
Colors: Gray Brown Red Black
Luster: Dull
Transparency: Opaque
Rarity: Common
Uses: Industrial, Construction, Architectural
Host rock: sedimentary basins
Texture: Not laminated or fissile
Grain size: Grains under 0.063 mm
Composition: Mostly silicate grains, a mix of silt- and clay-sized particles

Where rockhounds find mudstone

Classic worldwide localities

Global
United States
United Kingdom
China
Germany

Field-hunting tip

Where it forms: sedimentary basins. If you start seeing quartz, clay minerals, calcite in float, you are in the right ground.

Common questions

How do you identify mudstone?+

Mohs hardness is 2-3. Texture: Not laminated or fissile. Composition: Mostly silicate grains, a mix of silt- and clay-sized particles. Luster: dull. Common colors include gray, brown, red, black.

Where is mudstone found?+

Notable localities include Global; United States; United Kingdom; China; Germany.

What rocks look like mudstone?+

Mudstone is most often confused with Shale, Siltstone, Claystone. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with mudstone?+

Mudstone commonly co-occurs with Quartz, Clay minerals, Calcite, Pyrite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does mudstone form in?+

Where it forms: sedimentary basins. Working float back to the host body is the standard way to chase a fresh occurrence.

What is mudstone used for?+

Mudstone is used in industrial, construction, architectural.

Find mudstone on the map

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