Identification guide · Sedimentary Rocks
How to Identify Shale
Shale is typically gray, black, brown, red, or greenish, fine-grained enough that individual particles cannot be seen without magnification, and displays a dull, earthy luster on broken surfaces. Its defining trait is fissility, the tendency to split into thin, flat sheets along bedding planes, which separates it from mudstone, a similar rock that breaks into blocky chunks instead. Hardness is low, generally around 2 to 3, since it is composed largely of soft clay minerals, and it often feels slightly greasy or smooth to the touch. Black, organic-rich shale may leave a dark smudge on the fingers.
Shale identification checklist
| Color | Gray, Black, Brown, Red, Greenish gray | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Dull to earthy (slightly sub-vitreous if micaceous) | Fresh surfaces shine like glass; a dull surface usually means weathering, so chip or wet a corner. |
| Transparency | Opaque | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 2–3 | A copper coin (Mohs 3.5) scratches it, but a fingernail does not. |
| Fracture | Fissile; splits into thin platy layers | |
| Specific gravity | 2.4–2.8 | Ordinary rock-like heft (specific gravity about 2.5999999999999996), similar to quartz. |
Shale look-alikes and how to tell them apart
Both sit near Mohs 2–3, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Shale vs Claystone →
Both sit near Mohs 2–3, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Shale vs Mudstone →
Siltstone is harder (Mohs 3.5) than Shale (Mohs 2–3), so Siltstone will scratch Shale and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not. Shale vs Siltstone →
Quartz is harder (Mohs 7) than Shale (Mohs 2–3), so Quartz will scratch Shale and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not.
Shale identification FAQ
How can you tell if a rock is Shale?
Check the color (gray, black, brown, and red), the dull to earthy (slightly sub-vitreous if micaceous) luster, a Mohs hardness of 2–3. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Shale?
Shale has a Mohs hardness of 2–3. A copper coin (Mohs 3.5) scratches it, but a fingernail does not.
What is the difference between Shale and Claystone?
Shale (Mohs 2–3, dull to earthy (slightly sub-vitreous if micaceous) luster) and Claystone (Mohs 2–3, dull, earthy luster) look alike, but hardness, streak and heft usually separate them. See the full Shale vs Claystone comparison for a side-by-side table.
Is Shale worth anything?
Entry-level Shale 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 Shale?
Shale is most often gray, black, brown, and red. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
