igneous

Obsidian

Obsidian (Volcanic Glass)

Hardness: 5-6 (Mohs); Color: Black with grey or white flow banding; Luster: Vitreous (glassy); Crystal Structure: Amorphous; Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6

Hardness: 5-6 (Mohs)
Color: Black with grey or white flow banding
Luster: Vitreous (glassy)

Identified April 11, 2026More igneous →

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Physical properties

Hardness: 5-6 (Mohs); Color: Black with grey or white flow banding; Luster: Vitreous (glassy); Crystal Structure: Amorphous; Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6

Formation & geological history

Formed through the rapid cooling of silica-rich (rhyolitic) lava, preventing crystal growth. It is relatively young geologically (typically Cenozoic) because it eventually devitrifies into quartz and feldspar over millions of years.

Uses & applications

Used for making scalpels, jewelry, semi-precious gemstones, and ornamental carvings. Historically used for arrowheads and blades.

Geological facts

Obsidian is often called 'nature's glass'. The greyish-white banding seen in this specimen is known as flow banding, caused by the stretching of inclusions as the lava flowed before solidifying.

Field identification & locations

Identify in the field by its distinctive conchoidal (shell-like) fracture, extremely sharp edges, and glassy texture. Look for it in volcanic regions like the American West (Glass Buttes, OR).

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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