Identification guide · Metamorphic Rocks
How to Identify Blueschist
Blueschist is usually blue, blue gray, and gray 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; usually pale gray. It is most often confused with Greenschist, Mylonite, and Amphibolite; the tests below separate them.
Blueschist identification checklist
| Color | Blue, Blue gray, Gray, Green, Black | 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; usually pale gray | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | Variable by mineral grains | |
| Fracture | Uneven to splintery | |
| Specific gravity | 3.1 | Heavier than it looks (specific gravity about 3.1). |
Blueschist look-alikes and how to tell them apart
Blueschist is harder (Mohs 5.5) than Greenschist (Mohs 4.5), so Blueschist will scratch Greenschist and not the other way round. A steel knife blade (Mohs 5.5) scratches it; a copper coin does not. Blueschist vs Greenschist →
Both sit near Mohs 5.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Blueschist vs Mylonite →
Both sit near Mohs 5.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Blueschist vs Amphibolite →
Both sit near Mohs 5.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
Field tips
- Look for blue to blue-gray mineral grains or a blue cast in the rock.
- Blueschist is usually less glassy than blue slag and less transparent than gem minerals.
- A schistose or foliated fabric can support the identification.
- Locality helps because blueschist is tied to subduction-related metamorphic belts.
Blueschist identification FAQ
How can you tell if a rock is Blueschist?
Check the color (blue, blue gray, gray, and green), the silky luster, a Mohs hardness of 5.5 and a variable; usually pale gray streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Blueschist?
Blueschist 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 Blueschist and Greenschist?
Blueschist (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 Blueschist vs Greenschist comparison for a side-by-side table.
Is Blueschist worth anything?
Entry-level Blueschist specimens sell for around $5, typical collector pieces about $30, and premium examples $150 or more depending on size, quality and locality.
What color is Blueschist?
Blueschist is most often blue, blue gray, gray, and green. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
