igneous
Obsidian
Obsidian (Volcanic Glass)
Hardness: 5-5.5 (Mohs scale), Color: Jet black to dark grey, Luster: Vitreous (glassy), Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35-2.6
- Hardness
- 5-5
Identified More igneous →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 5-5.5 (Mohs scale), Color: Jet black to dark grey, Luster: Vitreous (glassy), Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35-2.6
Formation & geological history
Obsidian is formed from rapidly cooling felsic lava that is high in silica. The cooling happens so quickly that crystals do not have time to grow, resulting in a volcanic glass. It is commonly found in quaternary volcanic regions worldwide.
Uses & applications
Historically used for making extremely sharp arrowheads, knives, and scrapers. In modern times, it is used for ornamental carvings, jewelry, and high-quality scalpel blades for surgery.
Geological facts
Obsidian can produce a cutting edge many times sharper than high-quality steel surgical scalpels. Because it is glass, it is chemically unstable and over time devitrifies into quartz and feldspar, which is why ancient obsidian is rarely found.
Field identification & locations
Identify in the field by its distinct glassy luster and conchoidal (shell-like) fracture patterns. It is commonly found near volcanic domes or flow margins. Collectors look for unique varieties like Snowflake or Mahogany obsidian.
