igneous

Obsidian

Obsidian (Volcanic Glass)

Hardness: 5–6 Mohs; Color: Jet black with brownish streaks; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.4

Hardness: 5–6 Mohs
Color: Jet black with brownish streaks
Luster: Vitreous/Glassy

Identified April 6, 2026More igneous →

Identify your own rocks.

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

App Store Google Play

Physical properties

Hardness: 5–6 Mohs; Color: Jet black with brownish streaks; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.4

Formation & geological history

Formed from the rapid cooling of silica-rich (felsic) lava which prevents crystal growth. Usually found in margin of rhyolitic lava flows. Most specimens are relatively young geologically (Cenozoic) because glass eventually devitrifies into crystals.

Uses & applications

Historically used for arrowheads and blades; currently used in jewelry (lapidary), as a gemstone, and in high-end surgical scalpel blades because it can be fractured to a molecularly thin edge.

Geological facts

Because it is a glass and not a mineral, it has no internal crystal structure. Mahogany obsidian (like this appears to be) contains inclusions of hematite or magnetite creating brown-to-red swirls.

Field identification & locations

Identify by its glass-like texture and sharp, curved (conchoidal) edges. Often found near volcanic regions like Glass Buttes, Oregon or the Yellowstone area. Collectors should look for unique banding and clarity.

Other igneous specimens

See all

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