Igneous

Obsidian

Volcanic Glass (SiO2, usually >70% silica content)

Hardness: 5-5.5 on Mohs scale; Color: typically jet black but can be brown, gray, or green; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4

Hardness: 5-5
Color: typically jet black but can be brown, gray, or green
Luster: Vitreous (glassy)

Identified March 25, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the rapid cooling of silicious lava, which prevents the growth of crystals. It is typically found in geologically recent volcanic areas (Cenozoic era) because it is metastable and eventually devitrifies into fine-grained crystals over millions of years.

Uses & applications

Historically used for making sharp weapons and cutting tools; currently used in modern surgery for high-precision scalpels, ornamental jewelry (beads, cabochons), and as a gemstone for collectors.

Geological facts

Because it has no crystal structure, obsidian fractures with extremely sharp edges—sometimes down to molecular thinness. Native Americans and other ancient civilizations used it to create arrowheads, spear points, and mirrors.

Field identification & locations

Identify in the field by its intense glassy sheen and characteristic curved 'clamshell-like' (conchoidal) fractures. Primarily found in volcanic regions of the Western USA (Oregon, California), Mexico, Iceland, and Italy (Lipari). Note: it is easily scratched by steel compared to quartz.

Other igneous specimens

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

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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

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral