igneous
Obsidian
Volcanic Glass (SiO2)
Hardness: 5-5.5 (Mohs scale), Color: Black to dark brown/green, Luster: Vitreous/Glassy, Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35-2.6
- Hardness
- 5-5
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Physical properties
Hardness: 5-5.5 (Mohs scale), Color: Black to dark brown/green, Luster: Vitreous/Glassy, Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35-2.6
Formation & geological history
Formed through the rapid cooling of silica-rich (rhyolitic) lava, which prevents the growth of crystals. Typically found in young volcanic regions (Quaternary period) because glass is unstable and devitrifies over millions of years.
Uses & applications
Historical use for sharp tools and weapons (arrowheads/scalpels), jewelry (silver-sheen, snowflake, or gold-sheen varieties), and modern surgical blades due to its ability to be fractured to a molecularly thin edge.
Geological facts
Obsidian breaks with a conchoidal fracture, creating curved edges that are sharper than even high-quality steel scalpels. It is technically a mineraloid rather than a mineral because it lacks an internal crystal structure.
Field identification & locations
Identify in the field by its distinct glassy luster and curved, razor-sharp fracture patterns. Common in the Western US, Iceland, Mexico, and Italy. Collectors should look for variations like 'Mahogany' or 'Rainbow' obsidian.
