Comparison · Igneous Rocks vs Minerals
Obsidian vs Onyx
Obsidian and Onyx are easy to confuse. Onyx is harder (Mohs 6.5–7) than Obsidian (Mohs 5.5), so Onyx will scratch Obsidian and not the other way round. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. Below is a side-by-side comparison of their properties and the quick tests that separate them.
Key differences between Obsidian and Onyx
- Onyx is harder (Mohs 6.5–7) than Obsidian (Mohs 5.5), so Onyx will scratch Obsidian and not the other way round. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will.
- Hold a thin edge to the light: Obsidian is translucent, Onyx is Translucent to opaque.
- They overlap in black, brown, and multicolored, but Obsidian also comes in green and Onyx also comes in white and red.
- Well-formed crystals give it away: Obsidian is amorphous, Onyx is Trigonal (cryptocrystalline).
Obsidian vs Onyx properties
| Property | Obsidian | Onyx |
|---|---|---|
| Type | Igneous Rocks | Minerals |
| Mohs hardness | 5.5 | 6.5–7 |
| Luster | vitreous | Waxy to vitreous |
| Streak | White | White |
| Transparency | translucent | Translucent to opaque |
| Specific gravity | 2.4 | 2.6–2.65 |
| Crystal system | amorphous | Trigonal (cryptocrystalline) |
| Common colors | Black, Brown, Green, Rainbow | Black and white banded, Brown and white banded, Red and white banded |
| Rarity | common | common |
| Typical specimen value | $5–$100+ | $2–$150+ |
Which is more valuable?
Typical specimens of Obsidian and Onyx trade in a similar range, around $20.
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 Onyx
Onyx is a banded variety of chalcedony, a cryptocrystalline form of quartz, distinguished from agate by its straight, parallel bands rather than agate's characteristically curved layers. Most commercial onyx sold in black and white banded slabs is actually dyed to enhance contrast, since naturally occurring true onyx with strong black-and-white banding is relatively uncommon. Chemically identical to other quartz varieties, onyx owes its distinctive look entirely to microscopic banding formed during slow silica deposition.
Obsidian vs Onyx FAQ
Is Obsidian the same as Onyx?
No. Obsidian is a igneous rock while Onyx is a mineral. Onyx is harder (Mohs 6.5–7) than Obsidian (Mohs 5.5), so Onyx will scratch Obsidian and not the other way round. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will.
How can you tell Obsidian from Onyx?
Onyx is harder (Mohs 6.5–7) than Obsidian (Mohs 5.5), so Onyx will scratch Obsidian and not the other way round. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. Hold a thin edge to the light: Obsidian is translucent, Onyx is Translucent to opaque. They overlap in black, brown, and multicolored, but Obsidian also comes in green and Onyx also comes in white and red.
Which is harder, Obsidian or Onyx?
Onyx is harder: Mohs 6.5–7 against Mohs 5.5 for Obsidian.
Which is more valuable, Obsidian or Onyx?
Typical specimens of Obsidian and Onyx trade in a similar range, around $20.
