Identification guide · Sedimentary Rocks
How to Identify Siltstone
Siltstone is usually gray, tan, and brown with a dull luster and is opaque. It has a Mohs hardness of 3.5. A steel knife blade (Mohs 5.5) scratches it; a copper coin does not. Its streak is variable; often light gray to brown. It is most often confused with Claystone, Mudstone, and Shale; the tests below separate them.
Siltstone identification checklist
| Color | Gray, Tan, Brown, Red, Green | 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 | 3.5 | A steel knife blade (Mohs 5.5) scratches it; a copper coin does not. |
| Streak | Variable; often light gray to brown | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | Variable by mineral grains | |
| Fracture | Uneven to blocky | |
| Specific gravity | 2.45 | Ordinary rock-like heft (specific gravity about 2.45), similar to quartz. |
Siltstone look-alikes and how to tell them apart
Siltstone is harder (Mohs 3.5) than Claystone (Mohs 2–3), so Siltstone will scratch Claystone and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not. Siltstone vs Claystone →
Siltstone is harder (Mohs 3.5) than Mudstone (Mohs 2–3), so Siltstone will scratch Mudstone and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not. Siltstone 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. Siltstone vs Shale →
Both sit near Mohs 3.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Siltstone vs Ironstone →
Field tips
- Look for a fine, dull rock that is gritty but not sandy.
- Siltstone is usually less fissile than shale.
- Common colors include gray, tan, brown, red, and green.
- Fresh broken edges can reveal subtle bedding or lamination.
Siltstone identification FAQ
How can you tell if a rock is Siltstone?
Check the color (gray, tan, brown, and red), the dull luster, a Mohs hardness of 3.5 and a variable; often light gray to brown streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Siltstone?
Siltstone has a Mohs hardness of 3.5. A steel knife blade (Mohs 5.5) scratches it; a copper coin does not.
What is the difference between Siltstone and Claystone?
Siltstone (Mohs 3.5, dull luster) and Claystone (Mohs 2–3, dull, earthy luster) look alike, but hardness, streak and heft usually separate them. See the full Siltstone vs Claystone comparison for a side-by-side table.
Is Siltstone worth anything?
Entry-level Siltstone 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 Siltstone?
Siltstone is most often gray, tan, brown, and red. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
