Comparison · Metamorphic Rocks vs Igneous Rocks
Anthracite vs Obsidian
Anthracite and Obsidian are easy to confuse. Obsidian is harder (Mohs 5.5) than Anthracite (Mohs 2.5), so Obsidian will scratch Anthracite and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not. Below is a side-by-side comparison of their properties and the quick tests that separate them.
Key differences between Anthracite and Obsidian
- Obsidian is harder (Mohs 5.5) than Anthracite (Mohs 2.5), so Obsidian will scratch Anthracite and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not.
- Streak color differs: Anthracite leaves a black to dark gray streak on unglazed porcelain, Obsidian leaves white.
- Luster is different: Anthracite is submetallic while Obsidian is vitreous.
- Obsidian feels noticeably heavier for its size (specific gravity 2.4 versus 1.5 for Anthracite).
- Hold a thin edge to the light: Anthracite is opaque, Obsidian is translucent.
- They overlap in black, but Anthracite also comes in gray and Obsidian also comes in brown, green, and multicolored.
Anthracite vs Obsidian properties
| Property | Anthracite | Obsidian |
|---|---|---|
| Type | Metamorphic Rocks | Igneous Rocks |
| Mohs hardness | 2.5 | 5.5 |
| Luster | submetallic | vitreous |
| Streak | Black to dark gray | White |
| Transparency | opaque | translucent |
| Specific gravity | 1.5 | 2.4 |
| Crystal system | varies | amorphous |
| Common colors | Black, Steel gray | Black, Brown, Green, Rainbow |
| Rarity | common | common |
| Typical specimen value | $1–$50+ | $5–$100+ |
Which is more valuable?
Obsidian is usually the more valuable of the two: a typical collector specimen runs about $20 against $8 for Anthracite, and premium Obsidian can reach $100 or more.
About Anthracite
Anthracite is a hard, shiny, high-carbon coal. It often looks black, compact, lightweight for its size, and glassy to submetallic.
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.
Anthracite vs Obsidian FAQ
Is Anthracite the same as Obsidian?
No. Anthracite is a metamorphic rock while Obsidian is a igneous rock. Obsidian is harder (Mohs 5.5) than Anthracite (Mohs 2.5), so Obsidian will scratch Anthracite and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not.
How can you tell Anthracite from Obsidian?
Obsidian is harder (Mohs 5.5) than Anthracite (Mohs 2.5), so Obsidian will scratch Anthracite and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not. Streak color differs: Anthracite leaves a black to dark gray streak on unglazed porcelain, Obsidian leaves white. Luster is different: Anthracite is submetallic while Obsidian is vitreous.
Which is harder, Anthracite or Obsidian?
Obsidian is harder: Mohs 5.5 against Mohs 2.5 for Anthracite.
Which is more valuable, Anthracite or Obsidian?
Obsidian is usually the more valuable of the two: a typical collector specimen runs about $20 against $8 for Anthracite, and premium Obsidian can reach $100 or more.
