Identification guide · Metamorphic Rocks
How to Identify Skarn
Skarn is usually green, brown, and white with a dull 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 Amphibolite, Hornfels, and Metaconglomerate; the tests below separate them.
Skarn identification checklist
| Color | Green, Brown, White, Gray, Red, Black | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Dull | 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 | Variable by mineral grains | |
| Fracture | Uneven to granular | |
| Specific gravity | 3.1 | Heavier than it looks (specific gravity about 3.1). |
Skarn look-alikes and how to tell them apart
Both sit near Mohs 5.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Skarn vs Amphibolite →
Both sit near Mohs 5.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Skarn vs Hornfels →
Metaconglomerate is harder (Mohs 6.5) than Skarn (Mohs 5.5), so Metaconglomerate will scratch Skarn 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. Skarn vs Metaconglomerate →
Garnet is harder (Mohs 7) than Skarn (Mohs 5.5), so Garnet will scratch Skarn 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 a mixed calc-silicate rock with green pyroxene, epidote, garnet, or carbonate minerals.
- Skarn often forms near limestone or dolostone intruded by igneous rock.
- It can be colorful and granular rather than evenly layered.
- Do not identify a green rock as skarn unless the calc-silicate or contact-altered look is visible or locality supports it.
Skarn identification FAQ
How can you tell if a rock is Skarn?
Check the color (green, brown, white, and gray), the dull 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 Skarn?
Skarn 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 Skarn and Amphibolite?
Skarn (Mohs 5.5, dull luster) and Amphibolite (Mohs 5.5, dull luster) look alike, but hardness, streak and heft usually separate them. See the full Skarn vs Amphibolite comparison for a side-by-side table.
Is Skarn worth anything?
Entry-level Skarn specimens sell for around $2, typical collector pieces about $20, and premium examples $150 or more depending on size, quality and locality.
What color is Skarn?
Skarn is most often green, brown, white, and gray. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
