igneous

Mahogany Obsidian

Volcanic Glass (SiO2 + Fe2O3)

Hardness: 5-5.5 (Mohs scale); Color: Black with reddish-brown streaks or mottling; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.60

Hardness: 5-5
Color: Black with reddish-brown streaks or mottling
Luster: Vitreous/Glassy

Identified March 22, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of silica-rich (rhyolitic) lava. Rapid cooling prevents crystal growth. The mahogany color comes from inclusions of Hematite or Magnetite (iron oxides) that were trapped as the lava solidified.

Uses & applications

Used primarily for jewelry (cabochons and beads), ornamental carvings, and metaphysical collecting. Historically used for making sharp-edged tools like arrowheads and knives.

Geological facts

Because it is a glass and not a mineral, it is technically a 'mineraloid'. It has been used since prehistoric times for its ability to produce an edge significantly sharper than even the highest quality steel scalpels.

Field identification & locations

Identify in the field by its distinct conchoidal (shell-like) fracture edges and smooth, glassy surface. Commonly found in volcanic regions of the United States (Oregon, California, Utah), Mexico, and Japan. Collectors should look for high contrast between the red and black patterns.

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mineral

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