igneous

Obsidian

Volcanic glass (SiO2-rich amorphous silicate)

Hardness: 5–6 (Mohs scale); Color: Black, gray, or dark green with brownish fluidal swirls; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.4

Hardness: 5–6 (Mohs scale)
Color: Black, gray, or dark green with brownish fluidal swirls
Luster: Vitreous/glassy

Identified April 11, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of silica-rich (felsic) lava, which prevents crystals from growing. Usually found in volcanic regions from the Holocene to Pleistocene epochs.

Uses & applications

Used historically for sharp tools and weapons (arrowheads, blades). Today used in jewelry, ornamental carvings, and manufactured for high-end surgical scalpels.

Geological facts

Obsidian fractures with extremely sharp edges, much thinner and sharper than high-quality steel scalpels. The distinct swirls in this specimen are Flow Banding, caused by differences in composition or gas bubbles during lava flow.

Field identification & locations

Identify by its glassy texture, prominent conchoidal (shell-like) fractures, and sharp edges. Common in volcanic areas like the Western USA, Mexico, and Iceland.

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