igneous
Obsidian
Obsidian (Volcanic glass), SiO2 (major component)
Hardness: 5–6 (Mohs); Color: Black (jet black to smoky grey); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.3–2.6.
- Hardness
- 5–6 (Mohs)
- Color
- Black (jet black to smoky 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: Black (jet black to smoky grey); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.3–2.6.
Formation & geological history
Formed from extrusive igneous activity when highly felsic lava (rich in silica) cools extremely rapidly, preventing the growth of mineral crystals. Typically found in young volcanic regions (Neogene to Quaternary period) as it devitrifies/decomposes over geological time.
Uses & applications
Used historically for stone tools and weaponry (arrowheads, scalpels); currently used in jewelry (cabochons), surgical blades, and high-end ornamental décor.
Geological facts
Ancient Romans used it as mirrors. Because it breaks on the molecular level, obsidian blades are significantly sharper than stainless steel surgical scalpels. It is not technically a mineral because it lacks a crystalline structure.
Field identification & locations
Identify by its characteristic shell-like (conchoidal) fracture lines and glassy texture. Commonly found in Oregon, Mexico, Iceland, and Italy. Collectors should look for sharp edges and transparency in thin flakes.
