igneous
Black Obsidian
Volcanic Glass (SiO2 + impurities)
Hardness: 5–5.5 Mohs; Color: Jet black to dark gray; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (has conchoidal fracture); Specific Gravity: 2.4
- Hardness
- 5–5
- Color
- Jet black to dark gray
- Luster
- Vitreous/Glassy
Identified More igneous →
Explore Black 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: Jet black to dark gray; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (has conchoidal fracture); Specific Gravity: 2.4
Formation & geological history
Formed when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It is relatively young geologically because it eventually devitrifies into quartz and feldspar crystals.
Uses & applications
Used in jewelry making (beads and pendants), high-end surgical scalpels (for cleaner incisions), and as a decorative stone in home decor.
Geological facts
Ancient civilizations used obsidian to create sharp arrowheads and mirrors. Because it lacks a crystal structure, its edges can be thinned down to nearly a single molecule in thickness.
Field identification & locations
Identify by its glass-like appearance, razor-sharp edges if broken, and smooth shell-like (conchoidal) fractures. Commonly found in volcanic regions like Iceland, Mexico, and the Western United States.
