igneous

Obsidian

Volcanic Glass (predominantly SiO2)

Hardness: 5-5.5 (Mohs); Color: Jet black to dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4

Hardness: 5-5
Color: Jet black to dark brown
Luster: Vitreous/Glassy

Identified March 25, 2026More igneous →

Identify your own rocks.

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

App Store Google Play

Physical properties

Hardness: 5-5.5 (Mohs); Color: Jet black to dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4

Formation & geological history

Formed through the rapid cooling of felsic (silica-rich) lava, which prevents crystal growth. It occurs in geologically recent volcanic fields (often less than 10-20 million years old) due to its tendency to devitrify over time.

Uses & applications

Historically used for blade making and arrowheads; currently used in jewelry (beads/cabochons), scalpel blades for delicate surgery, and decorative ornamental carvings.

Geological facts

Because it is glass and not a mineral crystal, obsidian has no internal grain, allowing it to be fractured into blades only molecules thick, which are sharper than high-quality steel scalpels.

Field identification & locations

Identify by its glass-like appearance and distinctive curved, shell-like (conchoidal) fractures on the edges. Found in volcanic regions like the American West (Iceland, Mexico, Lipari). Collectors look for 'sheen' or inclusions like 'Snowflake' patterns.

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