igneous

Obsidian

Volcanic glass (SiO2)

Hardness: 5-6 Mohs. Color: Jet black to dark brown. Luster: Vitreous/glassy. Crystal structure: Amorphous. Cleavage: None (conchoidal fracture). Specific gravity: 2.35-2.6.

Hardness: 5-6 Mohs
Color: Jet black to dark brown
Luster: Vitreous/glassy

Identified May 29, 2026More igneous →

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Physical properties

Hardness: 5-6 Mohs. Color: Jet black to dark brown. Luster: Vitreous/glassy. Crystal structure: Amorphous. Cleavage: None (conchoidal fracture). Specific gravity: 2.35-2.6.

Formation & geological history

Formed by the rapid cooling of silica-rich (rhyolitic) lava, which prevents crystal growth. Often found in volcanic areas from the Cenozoic era.

Uses & applications

Used for making extremely sharp surgical scalpels, decorative jewelry, and ornamental carvings. Historically used for arrowheads and scrapers.

Geological facts

Unlike most rocks, obsidian is not made of crystals; it is a true volcanic glass. It breaks with extremely sharp edges that can be even finer than a steel blade.

Field identification & locations

Identify in the field by its distinct glassy luster and conchoidal (shell-like) fracture patterns. Commonly found in areas with recent volcanic activity like Oregon, Iceland, or Mexico.

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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