igneous

Obsidian

Obsidian (principally SiO2)

Hardness: 5-6 (Mohs scale); Color: Black, sometimes dark brown or green; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6.

Hardness: 5-6 (Mohs scale)
Color: Black, sometimes dark brown or green
Luster: Vitreous/Glassy

Identified March 18, 2026More igneous →

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Physical properties

Formation & geological history

Formed from rapid cooling of felsic (silica-rich) lava, which prevents crystal growth. It is relatively young geologically (mostly Cenozoic) because it eventually devitrifies into crystalline rock over millions of years.

Uses & applications

Used for surgical scalpel blades, jewelry (cabochons and beads), ornamental carvings, and ancient tools like arrowheads and knives.

Geological facts

Because it heals to a molecularly thin edge, obsidian scalpels are sharper than high-quality steel. It was highly prized by ancient civilizations like the Aztecs and Greeks for tools and mirrors.

Field identification & locations

Identify in the field by its characteristic curved (conchoidal) fracture lines and glassy texture. Commonly found in volcanic regions such as the American Northwest, Mexico, and Iceland. Collectors should look for 'flow' patterns or 'sheen'.

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