Hornfels is a fine-grained, hard, non-foliated metamorphic rock created through contact metamorphism where heat from igneous intrusions bakes the surrounding country rock. It typically appears as a dull, dense, and tough stone with a splintery fracture, often displaying small visible crystals like andalusite or cordierite that grew during the heating process.
Is this hornfels?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch hornfels with a known reference. Hornfels sits at Mohs 5-7, softer than the next harder reference, harder than the previous one.
- 2Read the textureLook at a fresh break with a hand lens. Texture: Non-foliated; grains fit closely together like a mosaic, sometimes with larger porphyroblasts. Grain size: Mostly fine-grained.
- 3Pick out the mineralsComposition: Commonly abundant small biotite crystals, with porphyroblasts of cordierite, garnet or andalusite. Identify the two or three most abundant grains.
- 4Read the lusterHold the specimen under a strong light. Luster: dull.
- 5Match the color rangeCompare against the expected color range: gray, dark gray, black, brown, greenish.
Often confused with
Hornfels vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside hornfels
Minerals reported to co-occur with hornfels. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Mohs hardness
- 5-7
- Density
- 2.6-3.0 g/cm³
- Luster
- Dull
- Transparency
- Opaque
- Rarity
- Common
- Uses
- Educational, Collector, Construction
- Host rock
- contact metamorphic zones
- Texture
- Non-foliated; grains fit closely together like a mosaic, sometimes with larger porphyroblasts
- Grain size
- Mostly fine-grained
- Composition
- Commonly abundant small biotite crystals, with porphyroblasts of cordierite, garnet or andalusite
Where rockhounds find hornfels
1 mapped spotsClassic worldwide localities
- United Kingdom
- Germany
- United States
- France
Field-hunting tip
Where it forms: contact metamorphic zones. If you start seeing andalusite, cordierite, biotite in float, you are in the right ground. In the U.S., the densest reported localities are in New Jersey. Start trip planning there.
Common questions
How do you identify hornfels?+
Where is hornfels found?+
Can I find hornfels in the United States?+
What rocks look like hornfels?+
What minerals are found with hornfels?+
What kind of rock does hornfels form in?+
What is hornfels used for?+
Find hornfels on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
