Igneous
Obsidian
Volcanic glass (SiO2 - Silicon Dioxide with impurities)
Hardness: 5-6 (Mohs); Color: Jet black to dark grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal (shell-like curves); Specific Gravity: 2.4
- Hardness
- 5-6 (Mohs)
- Color
- Jet black to dark grey
- Luster
- Vitreous/Glassy
Identified More igneous →
Explore Obsidian in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 5-6 (Mohs); Color: Jet black to dark grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal (shell-like curves); Specific Gravity: 2.4
Formation & geological history
Formed from the extremely rapid cooling of felsic (silica-rich) lava, which prevents crystal growth. It is primarily found in geologically recent volcanic areas, typically from the Cenozoic era.
Uses & applications
Used in the manufacture of extremely sharp surgical scalpels, decorative jewelry (cabochons and spheres), and historical stone tools (arrowheads).
Geological facts
Obsidian fractures with edges so sharp that they can be thin enough to cut through individual cells; because it is a glass and not a mineral, it is technically a 'mineraloid'.
Field identification & locations
Identify by its characteristic glassy sheen and razor-sharp conchoidal fractures. Commonly found in Oregon (USA), Mexico, and Iceland. Collectors look for a uniform black surface or 'sheen' effect when polished into a sphere.
