igneous

Mahogany Obsidian

Obsidian (volcanic glass with Hematite/Magnetite inclusions)

Hardness: 5-5.5 (Mohs scale); Color: Black with reddish-brown streaks or mottling; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.3 - 2.6.

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

Identified March 6, 2026More igneous →

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Physical properties

Formation & geological history

Formed when rhyolitic magma cools rapidly upon reaching the Earth's surface, preventing crystal growth. The mahogany color is specifically caused by high concentrations of iron oxide (Hematite or Magnetite) within the molten glass during solidification.

Uses & applications

Used primarily in lapidary work for cabochons, carvings, and beads. Historically used for sharp tools/knives. In modern times, it is used for jewelry, metaphysical palm stones, and decorative objects.

Geological facts

Mahogany obsidian is essentially natural glass. Despite its dark appearance, it is often chemically more similar to granite than basalt. Its edges can be sharp down to a molecular level, and it has been used in antiquity for surgical scalpels.

Field identification & locations

Identify in the field by its distinct glassy texture and scallop-shaped (conchoidal) fractures. The 'mahogany' patches are irregular. Common locations include Oregon and California (USA), 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)

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