igneous

Mahogany Obsidian

Volcanic glass containing Hematite or Magnetite (SiO2 + Fe2O3)

Hardness: 5-5.5 on Mohs scale; Color: Black with reddish-brown streaks or mottling; Luster: Vitreous (glassy); Crystal Structure: Amorphous; Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6

Hardness: 5-5
Color: Black with reddish-brown streaks or mottling
Luster: Vitreous (glassy)

Identified March 17, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of rhyolitic lava which prevents crystallization. The mahogany color is produced by high concentrations of iron oxides like hematite or magnetite trapped within the glass. Found in volcanic areas with high-silica eruptions, often dating from the Tertiary to Quaternary periods.

Uses & applications

Commonly used in jewelry (beads, cabochons), ornamental carvings, and metaphysical practices. Historically used to make sharp cutting tools and arrowheads.

Geological facts

Obsidian is technically not a mineral but a 'mineraloid' because it lacks a crystalline structure. The reddish-brown 'mahogany' patterns are often flow-banded, representing the movement of the lava as it cooled.

Field identification & locations

Identify by its glassy texture and distinctive reddish-brown swirls against a black background. It always exhibits conchoidal (shell-like) fracturing. Primarily found in the western United States (Oregon, California, Utah), Mexico, and volcanic regions of Japan.

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