igneous

Obsidian

Volcanic Glass (SiO2)

Hardness: 5–5.5 (Mohs), Color: Typically black but can be green or brown, Luster: Vitreous/Glassy, Crystal Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35–2.6

Hardness: 5–5

Identified April 12, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the rapid cooling of silica-rich (felsic) lava, which prevents the growth of crystals. Predominantly found in volcanic areas from various geological ages, primarily the Cenozoic era.

Uses & applications

Used historically for sharp tools/weapons (scalpels, arrowheads), in jewelry (beads, cabochons), and as an ornamental stone in modern decor.

Geological facts

Obsidian breaks with a conchoidal fracture, creating edges thinner than even the best steel scalpels. It is sometimes used in specialized eye surgeries today because it causes less tissue damage than metal blades.

Field identification & locations

Identify by its intense glassy sheen and characteristic conchoidal (curved, shell-like) fractures. Common in volcanic regions like the American West (Glass Buttes), Iceland, and Italy. Collectors look for unique varieties like Snowflake or Mahogany obsidian.

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