igneous

Obsidian

Obsidian (Volcanic glass), 70-75% SiO2

Hardness: 5-5.5 (Mohs scale); Color: Black to dark brown with translucent edges; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4

Hardness: 5-5
Color: Black to dark brown with translucent edges
Luster: Vitreous/glassy

Identified March 31, 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 scale); Color: Black to dark brown with translucent edges; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4

Formation & geological history

Formed from extrusive igneous activity where high-silica lava cools extremely rapidly, preventing crystal growth. Most specimens date from the Cenozoic era due to glass devitrification over geological time.

Uses & applications

Used historically for sharp cutting tools (scalpels, arrowheads), modern surgical blades, ornamental jewelry, and holistic healing collections.

Geological facts

Obsidian was prized in the Neolithic era for trading. Because it has no crystalline structure, it can be fractured to produce an edge only molecules thick, which is sharper than high-quality steel.

Field identification & locations

Identify by its distinctive glassy appearance and sharp, curved conchoidal fracture patterns. Found in volcanic regions like the American West, Iceland, and Mexico. Collectors look for unique varieties like Snowflake or Mahogany obsidian.

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