Identification guide · Sedimentary Rocks
How to Identify Arkose
Arkose is usually pink, red, and tan 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 variable; usually light-colored. It is most often confused with Sandstone, Graywacke, and Quartz; the tests below separate them.
Arkose identification checklist
| Color | Pink, Red, Tan, Gray, White | 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 | Variable; usually light-colored | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | Variable by mineral grains | |
| Fracture | Granular | |
| Specific gravity | 2.45 | Ordinary rock-like heft (specific gravity about 2.45), similar to quartz. |
Arkose 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. Arkose vs Sandstone →
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Arkose vs Graywacke →
Quartz is harder (Mohs 7) than Arkose (Mohs 6), so Quartz will scratch Arkose 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. Arkose 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 sand-sized grains with abundant pale, pink, or salmon feldspar.
- Arkose is usually more feldspar-rich and less uniformly quartz-rich than typical sandstone.
- Granular texture and weak bedding can help separate it from granite.
- Fresh surfaces show the grain mix better than weathered surfaces.
Arkose identification FAQ
How can you tell if a rock is Arkose?
Check the color (pink, red, tan, and gray), the dull luster, a Mohs hardness of 6 and a variable; usually light-colored streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Arkose?
Arkose 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 Arkose and Sandstone?
Arkose (Mohs 6, dull luster) and Sandstone (Mohs 6, dull luster) look alike, but hardness, streak and heft usually separate them. See the full Arkose vs Sandstone comparison for a side-by-side table.
Is Arkose worth anything?
Entry-level Arkose 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 Arkose?
Arkose is most often pink, red, tan, and gray. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
