igneous

Mahogany Obsidian

Obsidian (Volcanic Glass with Hematite/Magnetite inclusions)

Hardness: 5-5.5 (Mohs scale); Color: Black with reddish-brown streaks/swirls; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.35-2.6

Hardness: 5-5
Color: Black with reddish-brown streaks/swirls
Luster: Vitreous (glassy)

Identified May 26, 2026More igneous →

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Physical properties

Formation & geological history

Formed when felsic lava cools rapidly with minimal crystal growth. The mahogany color comes from inclusions of Magnetite or Hematite (iron oxides) during the cooling process. Most specimens are relatively young geologically, typically from the Tertiary or Quaternary periods.

Uses & applications

Used primarily for jewelry (cabochons, beads), decorative carvings, and metaphysical collecting. Historically used for sharp tools and arrowheads.

Geological facts

Despite looking like a rock, mahogany obsidian is technically a volcanic glass because it lacks a crystalline structure. The 'mahogany' patterns are often unique to each stone, representing flow bands of iron-rich impurities.

Field identification & locations

Identify by its glass-like feel, smooth conchoidal (shell-like) fracture edges, and distinctive reddish-brown and black mottling. Frequently found in volcanic regions of the Western USA (Oregon, California, Utah) and Mexico.

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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.

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Magnetite

Iron(II,III) oxide (Fe3O4)

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