Metamorphic Rocks · uncommon
Mylonite
Mylonite is a fine-grained, streaky metamorphic rock formed by intense ductile shearing in fault zones. It can show stretched mineral bands and a strong fabric.
Also known as: Mylonitic rock, Ductile fault rock.
Mylonite properties at a glance
| Type | Metamorphic Rocks |
|---|---|
| Chemical formula | Fine-grained fault-zone metamorphic rock; composition varies |
| Mohs hardness | 5.5 |
| Luster | silky |
| Streak | Variable by mineral content |
| Cleavage | Foliated to variable |
| Fracture | Uneven to splintery |
| Specific gravity | 2.7 |
| Transparency | opaque |
| Common colors | Gray, Black, White, Green, Brown |
| Rarity | uncommon |
Geology & background
Forms when rocks are intensely sheared and recrystallized in ductile fault zones, reducing grain size and producing a streaky or foliated fabric.
Mylonite is important for studying fault zones, mountain belts, and ductile deformation deep in the crust.
How to identify Mylonite
- Look for streaky, smeared, or strongly foliated fine-grained texture.
- Eye-shaped feldspar or quartz grains in a finer matrix can support mylonite.
- Mylonite is tied to fault-zone deformation rather than ordinary sedimentary bedding.
- Close-up photos of fabric are especially helpful for this name.
Full Mylonite identification guide: checklist, home tests and look-alikes →
Mylonite vs similar stones
Not sure it isn't something else? Side-by-side comparisons with the stones most often confused with Mylonite.
Where Mylonite is found
Reported from multiple metamorphic terrains worldwide; verify locality against specimen provenance when available.
How much is Mylonite worth?
| Entry-level specimen | $1 |
|---|---|
| Typical collector specimen | $10 |
| Premium specimen | $75+ |
Mylonite is usually valued for teaching, field context, locality, texture, mineral content, or unusually attractive specimens rather than high gem value.
Common uses
- Field identification
- Fault-zone study
- Structural geology
- Education
Interesting facts
- Mylonite records deep fault-zone deformation.
- Large original grains may survive as eye-shaped porphyroclasts.
- Its streaky fabric can reveal the direction of ancient shearing.
Care & safety
Mylonite can vary in toughness, cleavage, and weathering depending on mineral mix and fabric.
Avoid inhaling dust. Do not cut, grind, or polish unknown metamorphic rocks without eye protection and a properly rated respirator.
Mylonite FAQ
What is Mylonite?
Mylonite is a fine-grained, streaky metamorphic rock formed by intense ductile shearing in fault zones. It can show stretched mineral bands and a strong fabric.
How much is Mylonite worth?
Entry-level Mylonite specimens sell for around $1, a typical collector specimen for about $10, and premium pieces for $75 or more. Mylonite is usually valued for teaching, field context, locality, texture, mineral content, or unusually attractive specimens rather than high gem value.
How hard is Mylonite?
Mylonite has a Mohs hardness of 5.5. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will.
What is Mylonite used for?
Mylonite is used for field identification, fault-zone study, structural geology, and education.
