igneous
Obsidian
Obsidian (Amorphous volcanic glass)
Hardness: 5-6 (Mohs); Color: Black (sometimes with brown or sheen); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.3-2.6
- Hardness
- 5-6 (Mohs)
- Color
- Black (sometimes with brown or sheen)
- Luster
- Vitreous/Glassy
Identified More igneous →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 5-6 (Mohs); Color: Black (sometimes with brown or sheen); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.3-2.6
Formation & geological history
Formed when extrusive felsic lava cools rapidly with minimal crystal growth. Often found in geologically young volcanic areas (Quaternary age or later) where silica-rich lava hits water or air.
Uses & applications
Used in gemstone jewelry, ornamental carvings, high-end surgical scalpels for smooth incisions, and historical production of sharp tools and weapons.
Geological facts
Because it is glass and not a true mineral, it is technically unstable over millions of years and will eventually devitrify into a crystalline rock. Ancient civilizations used it as a trade commodity similar to currency.
Field identification & locations
Identify by its characteristic conchoidal (shell-like) fracture which creates razor-sharp edges. Commonly found in volcanic regions like the American West (Glass Buttes), Mexico, and Iceland.
More like this
Other igneous specimens
Granite
Granite
Igneous
Granite
Granite
Igneous
Blue Apatite
Fluorapatite (Ca5(PO4)3F)
mineral
Dalmatian Jasper
Aplite with Arfvedsonite inclusions
igneous
Magnetite
Iron(II,III) oxide (Fe3O4)
mineral
Dark Igneous Rock (Likely Basalt or Diabase)
Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.
Igneous