igneous

Obsidian

Volcanic Glass (SiO2 + impurities)

Hardness: 5–5.5 on Mohs scale; Color: typically jet black, can be dark green or brown; Luster: vitreous (glassy); Crystal structure: amorphous (none); Cleavage: none (conchoidal fracture); Specific gravity: 2.35–2.6.

Hardness: 5–5
Color: typically jet black, can be dark green or brown
Luster: vitreous (glassy)

Identified April 2, 2026More igneous →

Identify your own rocks.

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

App Store Google Play

Physical properties

Formation & geological history

Formed from extrusive igneous activity when highly viscous lava (rich in silica) cools extremely rapidly, preventing crystal growth. Usually found in geologically young volcanic areas (Cenozoic to Holocene).

Uses & applications

Used historically for sharp stone tools and weapons (arrowheads, scalpels). Modern uses include jewelry, decorative sculptures, and high-precision surgical scalpel blades.

Geological facts

Because it is glass, obsidian breaks with very sharp edges (conchoidal fracture). It is metastable and over millions of years will eventually devitrify into fine-grained quartz crystals, which is why ancient obsidian from the Paleozoic era is rare.

Field identification & locations

Identify by its incredible glass-like appearance, lack of grain, and sharp, curved fracture patterns. Commonly found near volcanic sites like the Cascades in the US, Iceland, and Mexico.

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