igneous

Mahogany Obsidian

Volcanic Glass (predominantly SiO2 with Fe2O3 inclusions)

Hardness: 5–5.5 Mohs scale; Color: Black with reddish-brown streaks or mottling; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); 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 April 12, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the rapid cooling of felsic (rhyolitic) lava, which prevents crystal growth. The mahogany coloration is caused by inclusions of hematite or magnetite (iron oxides) within the volcanic glass.

Uses & applications

Used in the lapidary arts for gemstones, beads, and carvings. Historically used for making sharp tools like arrowheads and knives. Also popular in metaphysical and collecting circles.

Geological facts

Obsidians are technically 'mineraloids' rather than minerals because they lack a crystalline structure. Mahogany obsidian is chemically unstable over millions of years, eventually 'devitrifying' into a stony rock.

Field identification & locations

Identify in the field by its distinct conchoidal (shell-like) fracture patterns and glassy texture. Commonly found in areas of recent volcanic activity such as the Western United States (Oregon, California), Mexico, and 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

Blue Apatite

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

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral