Identification guide · Metamorphic Rocks
How to Identify Mylonite
Mylonite is usually gray, black, and white with a silky luster and is opaque. It 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. Its streak is variable by mineral content. It is most often confused with Greenschist, Blueschist, and Quartz; the tests below separate them.
Mylonite identification checklist
| Color | Gray, Black, White, Green, Brown | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Silky | Compare a fresh surface with glass and metal; this luster is softer than either. |
| Transparency | Opaque | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 5.5 | It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. |
| Streak | Variable by mineral content | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | Foliated to variable | |
| Fracture | Uneven to splintery | |
| Specific gravity | 2.7 | Ordinary rock-like heft (specific gravity about 2.7), similar to quartz. |
Mylonite look-alikes and how to tell them apart
Mylonite is harder (Mohs 5.5) than Greenschist (Mohs 4.5), so Mylonite will scratch Greenschist and not the other way round. A steel knife blade (Mohs 5.5) scratches it; a copper coin does not. Mylonite vs Greenschist →
Both sit near Mohs 5.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Mylonite vs Blueschist →
Quartz is harder (Mohs 7) than Mylonite (Mohs 5.5), so Quartz will scratch Mylonite and not the other way round. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. Mylonite vs Quartz →
Dalmatian Jasper is harder (Mohs 7) than Mylonite (Mohs 5.5), so Dalmatian Jasper will scratch Mylonite and not the other way round. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will.
Field tips
- 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.
Mylonite identification FAQ
How can you tell if a rock is Mylonite?
Check the color (gray, black, white, and green), the silky luster, a Mohs hardness of 5.5 and a variable by mineral content streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of 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 the difference between Mylonite and Greenschist?
Mylonite (Mohs 5.5, silky luster) and Greenschist (Mohs 4.5, silky luster) look alike, but hardness, streak and heft usually separate them. See the full Mylonite vs Greenschist comparison for a side-by-side table.
Is Mylonite worth anything?
Entry-level Mylonite specimens sell for around $1, typical collector pieces about $10, and premium examples $75 or more depending on size, quality and locality.
What color is Mylonite?
Mylonite is most often gray, black, white, and green. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
