Identification guide · Sedimentary Rocks
How to Identify Graywacke
Graywacke is usually dark gray, greenish gray, and brown with a dull luster and is opaque. It has a Mohs hardness of 6. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. Its streak is gray to dark gray. It is most often confused with Sandstone, Arkose, and Quartz; the tests below separate them.
Graywacke identification checklist
| Color | Dark gray, Greenish gray, Brown, Black | 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 | 6 | It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. |
| Streak | Gray to dark gray | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | Variable by mineral grains | |
| Fracture | Granular to uneven | |
| Specific gravity | 2.55 | Ordinary rock-like heft (specific gravity about 2.55), similar to quartz. |
Graywacke look-alikes and how to tell them apart
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Graywacke vs Sandstone →
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Graywacke vs Arkose →
Quartz is harder (Mohs 7) than Graywacke (Mohs 6), so Quartz will scratch Graywacke and not the other way round. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. Graywacke vs Quartz →
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
Field tips
- Look for dark gray sandstone with mixed grain sizes and a muddy-looking matrix.
- Graywacke is usually less clean and less pale than quartz sandstone.
- It may show bedding or graded layers in field specimens.
- The rock can be quite tough compared with softer mudrocks.
Graywacke identification FAQ
How can you tell if a rock is Graywacke?
Check the color (dark gray, greenish gray, brown, and black), the dull luster, a Mohs hardness of 6 and a gray to dark gray streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Graywacke?
Graywacke has a Mohs hardness of 6. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will.
What is the difference between Graywacke and Sandstone?
Graywacke (Mohs 6, dull luster) and Sandstone (Mohs 6, dull luster) look alike, but hardness, streak and heft usually separate them. See the full Graywacke vs Sandstone comparison for a side-by-side table.
Is Graywacke worth anything?
Entry-level Graywacke 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 Graywacke?
Graywacke is most often dark gray, greenish gray, brown, and black. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
