Identification guide · Metamorphic Rocks
How to Identify Hornfels
Hornfels is usually gray, black, and brown with a dull luster and is opaque. It has a Mohs hardness of 6. 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 Amphibolite, Skarn, and Metaconglomerate; the tests below separate them.
Hornfels identification checklist
| Color | Gray, Black, Brown, Green, Red, White | 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 | 6 | 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 | None to poor | |
| Fracture | Splintery to conchoidal | Broken surfaces show smooth, curved, shell-like scoops. |
| Specific gravity | 2.7 | Ordinary rock-like heft (specific gravity about 2.7), similar to quartz. |
Hornfels look-alikes and how to tell them apart
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Hornfels vs Amphibolite →
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Hornfels vs Skarn →
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Hornfels vs Metaconglomerate →
Andalusite is harder (Mohs 6.5–7.5) than Hornfels (Mohs 6), so Andalusite will scratch Hornfels 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 hard, dense, fine-grained rock with little obvious foliation.
- Splintery or conchoidal fracture can support hornfels over shale or slate.
- Spotted or knotty textures may occur in contact-metamorphosed rocks.
- Hornfels can resemble basalt or chert, so texture and geologic context matter.
Hornfels identification FAQ
How can you tell if a rock is Hornfels?
Check the color (gray, black, brown, and green), the dull luster, a Mohs hardness of 6 and a variable; usually pale gray streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Hornfels?
Hornfels has a Mohs hardness of 6. 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 Hornfels and Amphibolite?
Hornfels (Mohs 6, dull luster) and Amphibolite (Mohs 5.5, dull luster) look alike, but hardness, streak and heft usually separate them. See the full Hornfels vs Amphibolite comparison for a side-by-side table.
Is Hornfels worth anything?
Entry-level Hornfels 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 Hornfels?
Hornfels is most often gray, black, brown, and green. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
