Mylonite is a fine-grained, cohesive metamorphic rock formed by intense ductile deformation in shear zones deep within the Earth's crust. It is characterized by a strong foliation or lineation, often containing rotated porphyroclasts of harder minerals like feldspar within a fine-grained matrix. Collectors often look for these in tectonically active zones where rocks have been significantly crushed and sheared under high pressure.

Hardness

5-7

Mohs

Luster

Dull to Silky

Streak

White

Transparency

Opaque

metamorphiccommon

Mylonite

Hardness: 5-7
Luster: Dull to Silky
Streak: White
Transparency: Opaque

Is this mylonite?

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 mylonite with a known reference. Mylonite sits at Mohs 5-7 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Mylonite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Mylonite typically shows a dull to silky luster.
4
Match the color range
Compare against the expected color range: gray, black, white, brown.
5
Look at form & habit
Typical habit: fine-grained, foliated, mylonitic texture.

Often confused with

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

metamorphic

Schist

Mohs 3-6

How to tell apart: Luster reads dull to silky on Mylonite and pearly on Schist.

Compare →

Often found alongside mylonite

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

All properties

Mohs hardness: 5-7
Density: 2.6-2.8 g/cm³
Colors: Gray Black White Brown
Streak: White
Luster: Dull to Silky
Transparency: Opaque
Crystal habit: Fine-grained, Foliated, Mylonitic Texture
Cleavage: None
Rarity: Common
Uses: Geological Study, Educational
Host rock: Ductile Shear Zones
Typical price: $5-20 hand specimen

Where rockhounds find mylonite

Classic worldwide localities

Moine Thrust Zone (Scotland)
San Andreas Fault (USA)
Alpine Fault (New Zealand)
Norway
Himalayas

Field-hunting tip

Look in ductile shear zones country — that is the host setting where mylonite typically forms. If you start seeing quartz, feldspar, mica in float, you are in the right ground. Field specimens usually show a fine-grained, foliated, mylonitic texture habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify mylonite?+

Mohs hardness is 5-7. It typically shows a dull to silky luster. The streak is white. Common colors include gray, black, white, brown.

Where is mylonite found?+

Notable localities include Moine Thrust Zone (Scotland); San Andreas Fault (USA); Alpine Fault (New Zealand); Norway; Himalayas.

How much is mylonite worth?+

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

What rocks look like mylonite?+

Mylonite is most often confused with Schist. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with mylonite?+

Mylonite commonly co-occurs with Quartz, Feldspar, Mica, Garnet. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does mylonite form in?+

Mylonite typically forms in ductile shear zones. Working float back to the host body is the standard way to chase a fresh occurrence.

What is mylonite used for?+

Mylonite is used in geological study, educational.

Find mylonite on the map

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