Obsidian (specifically Mahogany Obsidian)

Physical Properties

Hardness: 5-5.5 on Mohs scale; Color: Black with reddish-brown (hematite or magnetite) swirls; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4

Formation & Geological History

Formed through the rapid cooling of silica-rich felsic lava, which prevents crystal growth. It is chemically similar to rhyolite but has a glassy texture. Most common in geologically recent volcanic areas.

Uses & Applications

Used in gemstone and jewelry production, ornamental carvings, and historically for sharp tools like arrowheads and scalpal blades due to its extremely sharp edges.

Geological Facts

Mahogany obsidian gets its name and color from inclusions of iron oxides like hematite. Because it has no crystal structure, it is considered a mineraloid rather than a true mineral.

Field Identification & Locations

Identify in the field by its distinct glassy luster and shell-like (conchoidal) fracture patterns. It is heavy and feels like glass. Common in Idaho, Oregon, and the mountainous western USA.

Identified on: 4/15/2026

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