Comparison · Igneous Rocks
Diabase vs Obsidian
Diabase and Obsidian are easy to confuse because both are igneous rocks. Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Below is a side-by-side comparison of their properties and the quick tests that separate them.
Key differences between Diabase and Obsidian
- Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
- Streak color differs: Diabase leaves a variable; test individual minerals streak on unglazed porcelain, Obsidian leaves white.
- Luster is different: Diabase is varies while Obsidian is vitreous.
- Hold a thin edge to the light: Diabase is opaque, Obsidian is translucent.
- They overlap in multicolored, but Obsidian also comes in black, brown, and green.
Diabase vs Obsidian properties
| Property | Diabase | Obsidian |
|---|---|---|
| Type | Igneous Rocks | Igneous Rocks |
| Mohs hardness | 6 | 5.5 |
| Luster | varies | vitreous |
| Streak | Variable; test individual minerals | White |
| Transparency | opaque | translucent |
| Specific gravity | 2.8 | 2.4 |
| Crystal system | varies | amorphous |
| Common colors | Various | Black, Brown, Green, Rainbow |
| Rarity | common | common |
| Typical specimen value | $1–$75+ | $5–$100+ |
Which is more valuable?
Obsidian is usually the more valuable of the two: a typical collector specimen runs about $20 against $10 for Diabase, and premium Obsidian can reach $100 or more.
About Diabase
Diabase is a dark, medium-grained mafic igneous rock, also called dolerite in many regions. It commonly forms in dikes and sills and sits texturally between basalt and gabbro.
About Obsidian
Obsidian is a naturally occurring volcanic glass formed from rapidly cooled lava. Its sharp edges made it invaluable for ancient tools and weapons.
Diabase vs Obsidian FAQ
Is Diabase the same as Obsidian?
No. Both are igneous rocks, but they are distinct rock types with different hardness, streak or luster. Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
How can you tell Diabase from Obsidian?
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Streak color differs: Diabase leaves a variable; test individual minerals streak on unglazed porcelain, Obsidian leaves white. Luster is different: Diabase is varies while Obsidian is vitreous.
Which is harder, Diabase or Obsidian?
Diabase is harder: Mohs 6 against Mohs 5.5 for Obsidian.
Which is more valuable, Diabase or Obsidian?
Obsidian is usually the more valuable of the two: a typical collector specimen runs about $20 against $10 for Diabase, and premium Obsidian can reach $100 or more.
