Mineral/Igneous (Volcanic Glass)

Mahogany Obsidian or Red Jasper

Silicon Dioxide with Iron Oxide (SiO2+Fe2O3)

Hardness: 5-7 (Mohs scale), Color: Reddish-brown with black inclusions, Luster: Vitreous to waxy, Crystal Structure: Amorphous (non-crystalline) or Microcrystalline, Cleavage: Conchoidal fracture, Specific Gravity: 2.35 - 2.6

Identified March 28, 2026More mineral/igneous (volcanic glass) →

Explore Mahogany Obsidian or Red Jasper in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed from the rapid cooling of silica-rich lava (rhyolitic) that cools too quickly for crystals to grow. Iron inclusions create the mahogany/reddish hues. Commonly found in Tertiary to Holocene volcanic fields.

Uses & applications

Used primarily in jewelry (cabochons, beads), decorative carvings, and alternative healing practices. Historically used for making sharp cutting tools like arrowheads and knives.

Geological facts

Mahogany Obsidian is unique because its needle-like inclusions of hematite or magnetite create the distinct reddish-brown swirls. It has been used by humans for over 10,000 years for weaponry due to its ability to fracture into razor-sharp edges.

Field identification & locations

Identify by its glassy luster and smooth, curved (conchoidal) fractures. Typically found in volcanic regions such as the American Northwest (Oregon, California), Mexico, and Japan. Collectors look for high contrast between the red and black patterns.

Other mineral/igneous (volcanic glass) specimens

See all

Obsidian

Extrusive igneous volcanic glass

igneous

Obsidian

Volcanic glass

igneous

Snowflake Obsidian

Obsidian with Cristobalite inclusions (SiO2)

igneous

Snowflake Obsidian

Spherulitic Obsidian (Amorphous Volcanic Glass with Cristobalite inclusions)

igneous

Snowflake Obsidian

Obsidian with Cristobalite inclusions

igneous

Obsidian

Volcanic glass

igneous