Identification guide · Metamorphic Rocks
How to Identify Amphibolite
Amphibolite is usually black, dark green, and gray 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; usually pale gray to white. It is most often confused with Hornfels, Skarn, and Metaconglomerate; the tests below separate them.
Amphibolite identification checklist
| Color | Black, Dark green, Gray, White, Red | 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; usually pale gray to white | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | Variable by amphibole and feldspar grains | |
| Fracture | Uneven to granular | |
| Specific gravity | 3 | Ordinary rock-like heft (specific gravity about 3), similar to quartz. |
Amphibolite 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. Amphibolite vs Hornfels →
Both sit near Mohs 5.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Amphibolite vs Skarn →
Metaconglomerate is harder (Mohs 6.5) than Amphibolite (Mohs 5.5), so Metaconglomerate will scratch Amphibolite 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. Amphibolite vs Metaconglomerate →
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 a dark green to black granular rock dominated by amphibole.
- Red garnets in a dark amphibole-rich matrix can support garnet amphibolite.
- Amphibolite is usually tougher and less strongly layered than mica schist.
- A fresh surface shows mineral grains better than a weathered dark rind.
Amphibolite identification FAQ
How can you tell if a rock is Amphibolite?
Check the color (black, dark green, gray, and white), the dull luster, a Mohs hardness of 5.5 and a variable; usually pale gray to white streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Amphibolite?
Amphibolite 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 Amphibolite and Hornfels?
Amphibolite (Mohs 5.5, dull luster) and Hornfels (Mohs 6, dull luster) look alike, but hardness, streak and heft usually separate them. See the full Amphibolite vs Hornfels comparison for a side-by-side table.
Is Amphibolite worth anything?
Entry-level Amphibolite 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 Amphibolite?
Amphibolite is most often black, dark green, gray, and white. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
