igneous
Obsidian
Obsidian (Volcanic Glass), primarily SiO2 (Silicon Dioxide)
Hardness: 5-5.5 (Mohs); Color: Black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- Black
- 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: 5-5.5 (Mohs); Color: Black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific Gravity: 2.4
Formation & geological history
Formed from extrusive igneous activity when felsic lava cools extremely rapidly, preventing the growth of mineral crystals. Typically found in young volcanic regions (Quaternary age) as it devitrifies over time.
Uses & applications
Used for making ultra-sharp surgical scalpels, decorative jewelry (cabochons), and ornamental carvings. Historically used for weapons like arrowheads and knives.
Geological facts
Obsidian fractures with shell-like (conchoidal) curves that create edges thinner and sharper than high-quality steel scalpels. It is technically a mineraloid rather than a mineral because it lacks a crystalline structure.
Field identification & locations
Identify by its jet-black color, intense glassy shine, and sharp, curved edges where broken. Commonly found in Oregon (Glass Buttes), California, and Mexico. Collectors look for translucency at the edges.
