Identification guide · Metamorphic Rocks
How to Identify Slate
Slate is usually gray, black, and green 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; test individual minerals. It is most often confused with Siltstone, Ironstone, and Amphibolite; the tests below separate them.
Slate identification checklist
| Color | Gray, Black, Green, Purple, Red | 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; test individual minerals | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | Perfect (slaty cleavage) | |
| Fracture | Flat | |
| Specific gravity | 2.8 | Ordinary rock-like heft (specific gravity about 2.8), similar to quartz. |
Slate look-alikes and how to tell them apart
Both sit near Mohs 3.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Slate vs Siltstone →
Both sit near Mohs 3.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Slate vs Ironstone →
Amphibolite is harder (Mohs 5.5) than Slate (Mohs 3.5), so Amphibolite will scratch Slate and not the other way round. A steel knife blade (Mohs 5.5) scratches it; a copper coin does not. Slate vs Amphibolite →
Greenschist is harder (Mohs 4.5) than Slate (Mohs 3.5), so Greenschist will scratch Slate and not the other way round. A steel knife blade (Mohs 5.5) scratches it; a copper coin does not.
Field tips
- Check whether the specimen's color and texture match typical Slate examples.
- Use hardness, streak, luster, grain size, and locality together; no single trait is enough for every specimen.
Slate identification FAQ
How can you tell if a rock is Slate?
Check the color (gray, black, green, and purple), the dull luster, a Mohs hardness of 3.5 and a variable; test individual minerals streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Slate?
Slate 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 Slate and Siltstone?
Slate (Mohs 3.5, dull luster) and Siltstone (Mohs 3.5, dull luster) look alike, but hardness, streak and heft usually separate them. See the full Slate vs Siltstone comparison for a side-by-side table.
Is Slate worth anything?
Entry-level Slate specimens sell for around $1, typical collector pieces about $10, and premium examples $100 or more depending on size, quality and locality.
What color is Slate?
Slate is most often gray, black, green, and purple. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
