igneous

Mahogany Obsidian

Obsidian (Volcanic Glass) with Hematite or Magnetite inclusions, SiO2 (Silicon Dioxide)

Hardness: 5–5.5 on Mohs scale; Color: Reddish-brown (mahogany) with black mottling; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35–2.6

Hardness: 5–5
Color: Reddish-brown (mahogany) with black mottling
Luster: Vitreous (glassy)

Identified March 17, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of silica-rich (rhyolitic) lava, which prevents crystal growth. The mahogany color is produced by high concentrations of iron oxides (hematite or magnetite) that oxidation creates during the cooling process. Most specimens are relatively young geologically (Cenozoic era).

Uses & applications

Commonly used for lapidary work such as jewelry (cabochons, beads), decorative carvings, and sphere making. In industry, it is historically significant for making ultra-sharp cutting tools like scalpels.

Geological facts

Obsidians have been used since prehistoric times for weapons. Mahogany obsidian is chemically similar to black obsidian but differs due to its iron content. It is known as a 'natural glass' and is technically not a mineral because it lacks an internal crystalline structure.

Field identification & locations

Identify in the field by its glassy texture and distinctive conchoidal (shell-like) fracture pattern. It is commonly found in areas of recent volcanic activity, specifically in the Western United States (Oregon, California), Mexico, and Japan.

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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)

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Magnetite

Magnetite (Fe3O4)

mineral