igneous

Obsidian

Volcanic Glass (SiO2)

Hardness: 5-6 (Mohs); Color: Black, deep brown, or grey; Luster: Vitreous/Glassy; Structure: Amorphous (no crystal structure); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6.

Hardness: 5-6 (Mohs)
Color: Black, deep brown, or grey
Luster: Vitreous/Glassy

Identified February 15, 2026More igneous →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Hardness: 5-6 (Mohs); Color: Black, deep brown, or grey; Luster: Vitreous/Glassy; Structure: Amorphous (no crystal structure); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6.

Formation & geological history

Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It is relatively young geologically (mostly Cenozoic) because it is unstable and eventually devitrifies into crystals.

Uses & applications

Used historically for tools/weapons (arrowheads, blades); modern uses include surgical scalpels, ornamental jewelry, cabochons, and decorative architecture.

Geological facts

Because obsidian lacks crystal structure, its fracture edges can reach almost molecular thinness, making it much sharper than high-quality steel scalpels. It remains a popular material for 'knapping' by modern hobbyists recreating ancient tools.

Field identification & locations

Identify in the field by its characteristic smooth, curved (conchoidal) fracture surfaces that look like broken glass. Commonly found in volcanic regions such as the American Northwest, Mexico, Iceland, and Japan.

Other igneous specimens

See all

Blue Apatite

Fluorapatite (Ca5(PO4)3F)

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