Identification guide · Igneous Rocks
How to Identify Diabase
Diabase is variable in color with a varies 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; test individual minerals. It is most often confused with Rainbow Obsidian, Obsidian, and Diorite; the tests below separate them.
Diabase identification checklist
| Luster | Varies | 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; test individual minerals | 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 granular | |
| Specific gravity | 2.8 | Ordinary rock-like heft (specific gravity about 2.8), similar to quartz. |
Diabase 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. Diabase vs Rainbow Obsidian →
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Diabase vs Obsidian →
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Diabase vs Diorite →
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, dense, medium-grained igneous texture.
- Diabase is also commonly called dolerite outside North America.
- It is usually coarser than basalt and finer than gabbro.
Diabase identification FAQ
How can you tell if a rock is Diabase?
Check the color (variable), the varies luster, a Mohs hardness of 6 and a variable; test individual minerals streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Diabase?
Diabase 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 Diabase and Rainbow Obsidian?
Diabase (Mohs 6, varies luster) and Rainbow Obsidian (Mohs 5.5, vitreous luster) look alike, but hardness, streak and heft usually separate them. See the full Diabase vs Rainbow Obsidian comparison for a side-by-side table.
Is Diabase worth anything?
Entry-level Diabase 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 Diabase?
Diabase is most often . Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
