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, 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.