igneous

Obsidian

Obsidian (principally SiO2 silicon dioxide)

Hardness: 5-5.5 on Mohs scale; Color: typically black but can be brown, green, or mahogany; Luster: vitreous (glassy); Crystal structure: amorphous (volcanic glass); Cleavage: none (conchoidal fracture); Specific gravity: 2.35-2.6

Hardness: 5-5
Color: typically black but can be brown, green, or mahogany
Luster: vitreous (glassy)

Identified April 11, 2026More igneous →

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Physical properties

Formation & geological history

Formed from rapid cooling of felsic (silicic) lava, which prevents the formation of crystal structures. Usually found in geologically young volcanic areas.

Uses & applications

Used for surgical scalpel blades, jewelry, decorative ornaments, and historically for arrowheads, spear points, and cutting tools.

Geological facts

Obsidian is technically a glass rather than a mineral because it lacks a crystalline structure. Ancient Aztecs used obsidian mirrors for divination, and it is still used in high-end medical surgery because it can be fractured to a thickness of only a few molecules.

Field identification & locations

Identified by its sharp, curved (conchoidal) fracture surfaces and intense glassy shine. Commonly found along the edges of rhyolitic lava flows in places like Iceland, Mexico, or the Western United States (e.g., Glass Buttes).

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

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

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

mineral

Magnetite

Magnetite (Fe3O4)

mineral