igneous

Obsidian

Volcanic Glass (Primary component: SiO2)

Hardness: 5–5.5 Mohs; Color: Jet black to dark brown; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like); Specific Gravity: 2.4

Hardness: 5–5
Color: Jet black to dark brown
Luster: Vitreous (glassy)

Identified March 16, 2026More igneous →

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Physical properties

Hardness: 5–5.5 Mohs; Color: Jet black to dark brown; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like); Specific Gravity: 2.4

Formation & geological history

Formed through the rapid cooling of felsic (silicic) lava, which prevents the growth of crystals. It is usually found in geologically young areas as it undergoes devitrification over time, rarely older than several million years.

Uses & applications

Used historically for stone tools and weaponry (arrowheads, scalpels); currently used in jewelry (gemstones), surgical blades, and as an ornamental stone in construction and decor.

Geological facts

Obsidian can produce a cutting edge that is thinner and sharper than high-quality steel scalpels. Famous varieties include Snowflake Obsidian (with spherulite inclusions) and Rainbow Obsidian.

Field identification & locations

Identify in the field by its distinct glassy appearance and sharp, curved conchoidal fracture edges. Found in volcanic regions like the American West (Oregon, California), Iceland, Japan, and Mexico. Collectors should look for unique banding or light-refracting inclusions.

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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