Comparison · Igneous Rocks vs Materials
Obsidian vs Slag Glass
Obsidian and Slag Glass are easy to confuse. Both sit near Mohs 5.5, 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 Obsidian and Slag Glass
- Both sit near Mohs 5.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
- Streak color differs: Obsidian leaves a white streak on unglazed porcelain, Slag Glass leaves white to colorless.
- They overlap in black, brown, and green, but Obsidian also comes in multicolored and Slag Glass also comes in blue and white.
Obsidian vs Slag Glass properties
| Property | Obsidian | Slag Glass |
|---|---|---|
| Type | Igneous Rocks | Materials |
| Mohs hardness | 5.5 | 5.5 |
| Luster | vitreous | vitreous |
| Streak | White | White to colorless |
| Transparency | translucent | translucent |
| Specific gravity | 2.4 | 2.5 |
| Crystal system | amorphous | amorphous |
| Common colors | Black, Brown, Green, Rainbow | Blue, Green, Black, Brown, White, Swirled |
| Rarity | common | common |
| Typical specimen value | $5–$100+ | $1–$80+ |
Which is more valuable?
Obsidian is usually the more valuable of the two: a typical collector specimen runs about $20 against $10 for Slag Glass, and premium Obsidian can reach $100 or more.
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.
About Slag Glass
Slag Glass is glassy industrial byproduct or manufactured glass with swirled colors, sometimes collected or knapped like stone.
Obsidian vs Slag Glass FAQ
Is Obsidian the same as Slag Glass?
No. Obsidian is a igneous rock while Slag Glass is a material. Both sit near Mohs 5.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
How can you tell Obsidian from Slag Glass?
Both sit near Mohs 5.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Streak color differs: Obsidian leaves a white streak on unglazed porcelain, Slag Glass leaves white to colorless. They overlap in black, brown, and green, but Obsidian also comes in multicolored and Slag Glass also comes in blue and white.
Which is harder, Obsidian or Slag Glass?
They are about equally hard, both near Mohs 5.5.
Which is more valuable, Obsidian or Slag Glass?
Obsidian is usually the more valuable of the two: a typical collector specimen runs about $20 against $10 for Slag Glass, and premium Obsidian can reach $100 or more.
