igneous

Mahogany Obsidian

Volcanic Glass with Hematite/Magnetite inclusions

Hardness: 5.0-5.5 (Mohs scale), Color: Black with reddish-brown streaks, Luster: Vitreous/Glassy, Structure: Amorphous (non-crystalline), Cleavage: None/Conchoidal fracture, Specific Gravity: 2.35-2.60

Hardness: 5

Identified May 3, 2026More igneous →

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Physical properties

Formation & geological history

Formed when rhyolitic lava cools rapidly with minimal crystal growth. The reddish-brown 'mahogany' color is caused by the inclusion of magnetite or hematite during the cooling process. Common in volcanic terrains from the Tertiary to Quaternary periods.

Uses & applications

Commonly used for lapidary work like cabochons, beads, and carvings. Historically used for making sharp tools, blades, and arrowheads due to its conchoidal fracture.

Geological facts

Unlike minerals, obsidian is a 'mineraloid' because it lacks an internal crystalline structure. It is technically a supercooled liquid. The reddish spots in Mahogany Obsidian are often referred to as 'spherulites' when they form rounded patterns.

Field identification & locations

Identify by its glass-like texture, sharp edges when broken, and characteristic reddish-brown swirls against a black base. Found frequently in the western United States, Mexico, and Japan. Collectors should look for high contrast between the red and black colors.

Other igneous specimens

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

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

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Blue Apatite

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

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral