Hornfels
Hornfels is a hard, dense, fine-grained contact metamorphic rock. It often looks dark, tough, and baked, with little visible layering.
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Hornfels properties at a glance
| Type | Metamorphic Rocks |
|---|---|
| Chemical formula | Fine-grained contact metamorphic rock; composition varies |
| Mohs hardness | 6 |
| Luster | dull |
| Streak | Variable; usually pale gray |
| Cleavage | None to poor |
| Fracture | Splintery to conchoidal |
| Specific gravity | 2.7 |
| Transparency | opaque |
| Common colors | Gray, Black, Brown, Green, Red, White |
| Rarity | common |
Geology & background
Forms when rocks are baked by nearby magma or hot intrusions, producing a hard, fine-grained contact metamorphic texture.
Hornfels is a classic marker of contact metamorphism around hot igneous bodies.
How to identify Hornfels
- 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.
Where Hornfels is found
Reported from multiple metamorphic terrains worldwide; verify locality against specimen provenance when available.
How much is Hornfels worth?
| Entry-level specimen | $1 |
|---|---|
| Typical collector specimen | $10 |
| Premium specimen | $75+ |
Hornfels is usually valued for teaching, field context, locality, texture, mineral content, or unusually attractive specimens rather than high gem value.
Common uses
- Field identification
- Contact metamorphism study
- Education
- Durable aggregate in some regions
Interesting facts
- Hornfels commonly forms next to igneous intrusions.
- It may break with a splintery or conchoidal fracture.
- Some hornfels contains spotted or knotty minerals from contact metamorphism.
Care & safety
Hornfels can vary in toughness, cleavage, and weathering depending on mineral mix and fabric.
Avoid inhaling dust. Do not cut, grind, or polish unknown metamorphic rocks without eye protection and a properly rated respirator.
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