Igneous

Mahogany Obsidian

Volcanic Glass with Hematite/Magnetite inclusions (SiO2 + Fe2O3)

Hardness: 5-5.5 on Mohs scale; Color: Black with reddish-brown streaks or splotches; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (Conchoidal fracture); Specific Gravity: 2.3-2.5

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

Identified March 25, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the rapid cooling of rhyolitic lava that is silica-rich. The mahogany color comes from the inclusion of iron oxides (hematite or magnetite) during the solidification process. It is found in volcanic regions of various ages, typically Cenozoic.

Uses & applications

Used primarily in lapidary arts for jewelry (beads, cabochons), ornamental carvings (like the mushroom shown), and metaphysical collecting. Historically used for making sharp tools and arrowheads.

Geological facts

Unlike crystalline rocks, obsidian is an 'amorphous' volcanic glass because it cooled so quickly that crystals didn't have time to grow. The 'mahogany' name refers to its resemblance to the dark, rich wood of the same name.

Field identification & locations

Identify by its glassy texture, shell-like (conchoidal) fractures, and the specific reddish-brown coloring mixed with black. Commonly found in the western USA (Oregon, California), Mexico, and Japan. Collectors should look for a smooth, glass-like surface and no visible grains.

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