igneous

Obsidian

Obsidian (principally SiO2)

Hardness: 5–5.5 Mohs scale; Color: Black, dark brown, or grey (rarely green, red, or iridescent); Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None, exhibits conchoidal fracture; Specific Gravity: 2.35–2.6.

Hardness: 5–5
Luster: Vitreous (glassy)

Identified February 13, 2026More igneous →

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Physical properties

Formation & geological history

Formed from rapid cooling of felsic (silica-rich) lava, which prevents the growth of crystals. It is typically found in volcanic areas of the Cenozoic era, as it eventually devitrifies into fine-grained minerals over millions of years.

Uses & applications

Used for making surgical scalpels due to its ability to create an edge thinner than steel, ornamental jewelry, carvings, and high-end abrasive tools. Historically prized for arrowheads and scrapers.

Geological facts

Known as 'nature's glass,' obsidian can be fractured to create an edge one molecule thick. The 'Snowflake' variety contains inclusions of spherulitic cristobalite (white spots).

Field identification & locations

Identify in the field by its deep black color and distinctive conchoidal (curved, shell-like) fractures with extremely sharp edges. Commonly found in areas with geologically recent volcanic activity like Oregon, Iceland, Mexico, and Japan.

Other igneous specimens

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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