igneous

Obsidian

Volcanic glass (Mainly SiO2)

Hardness: 5.0–5.5; Color: Jet black to dark brown; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35–2.6

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

Identified March 31, 2026More igneous →

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Physical properties

Hardness: 5.0–5.5; Color: Jet black to dark brown; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35–2.6

Formation & geological history

Formed from the rapid cooling of felsic (silica-rich) lava, which prevents crystal growth. Usually associated with rhyolitic volcanic eruptions. Most specimens are relatively young (Cenozoic) because the glass eventually devitrifies into crystals over millions of years.

Uses & applications

Used in gemstone and jewelry trade, ornamental carvings, high-precision surgical scalpel blades, and historically for arrowheads and scrapers.

Geological facts

Obsidians are so sharp that a scalpel blade can be narrowed to a single atom width, performing cuts with less tissue damage than steel. It is not a true mineral because it lacks a crystalline structure.

Field identification & locations

Identify by its glass-like appearance and smooth, curved (conchoidal) fracture surfaces similar to a broken glass bottle. Commonly found in areas of recent volcanic activity like Oregon, California (Glass Mountain), Iceland, and Mexico.

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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