igneous

Mahogany Obsidian

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

Hardness: 5-5.5 (Mohs); Color: Dark brown or reddish-brown with black streaks; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.3-2.6

Hardness: 5-5
Color: Dark brown or reddish-brown with black streaks
Luster: Vitreous/Glassy

Identified April 13, 2026More igneous →

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Physical properties

Hardness: 5-5.5 (Mohs); Color: Dark brown or reddish-brown with black streaks; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.3-2.6

Formation & geological history

Formed through the rapid cooling of felsic lava, preventing crystal growth. The reddish-brown color is caused by the inclusion of iron oxides like hematite or magnetite during the solidification process.

Uses & applications

Commonly used for jewelry (cabochons and beads), ornamental carvings, and by collectors. Historically used for making sharp tools and blades.

Geological facts

Obsidian is technically an 'igneous rock' but also a volcanic glass. Mahogany obsidian is prized for its unique undulating patterns that resemble the wood grain of mahogany trees.

Field identification & locations

Identify in the field by its distinct glassy texture and sharp, shell-like (conchoidal) fractures. Found in volcanic regions such as the American Northwest (Oregon), Mexico, and Japan.

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

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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)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Magnetite

Magnetite (Fe3O4)

mineral