Comparison · Semi-Precious Stones vs Materials
Opal vs Slag Glass
Opal 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 Opal 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: Opal leaves a white streak on unglazed porcelain, Slag Glass leaves white to colorless.
- They overlap in white, black, and blue, but Opal also comes in pink and Slag Glass also comes in green and brown.
Opal vs Slag Glass properties
| Property | Opal | Slag Glass |
|---|---|---|
| Type | Semi-Precious Stones | Materials |
| Mohs hardness | 5.5 | 5.5 |
| Luster | vitreous | vitreous |
| Streak | White | White to colorless |
| Transparency | translucent | translucent |
| Specific gravity | 2.1 | 2.5 |
| Crystal system | amorphous | amorphous |
| Common colors | White, Black, Fire, Blue, Pink | Blue, Green, Black, Brown, White, Swirled |
| Rarity | uncommon | common |
| Typical specimen value | $50–$10000+ | $1–$80+ |
Which is more valuable?
Opal is usually the more valuable of the two: a typical collector specimen runs about $500 against $10 for Slag Glass, and premium Opal can reach $10000 or more.
About Opal
Opal is famous for its play-of-color, caused by the diffraction of light through microscopic silica spheres. Australia produces 95% of the world's opals.
About Slag Glass
Slag Glass is glassy industrial byproduct or manufactured glass with swirled colors, sometimes collected or knapped like stone.
Opal vs Slag Glass FAQ
Is Opal the same as Slag Glass?
No. Opal is a semi-precious stone 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 Opal 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: Opal leaves a white streak on unglazed porcelain, Slag Glass leaves white to colorless. They overlap in white, black, and blue, but Opal also comes in pink and Slag Glass also comes in green and brown.
Which is harder, Opal or Slag Glass?
They are about equally hard, both near Mohs 5.5.
Which is more valuable, Opal or Slag Glass?
Opal is usually the more valuable of the two: a typical collector specimen runs about $500 against $10 for Slag Glass, and premium Opal can reach $10000 or more.
