igneous

Obsidian

Volcanic glass (no mineral formula due to amorphous structure, mostly SiO2)

Hardness: 5-5.5 (Mohs scale); Color: Black with dark grey banding; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (has conchoidal fracture); Specific Gravity: 2.35-2.60.

Hardness: 5-5
Color: Black with dark grey banding
Luster: Vitreous/Glassy

Identified March 29, 2026More igneous →

Identify your own rocks.

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

App Store Google Play

Physical properties

Formation & geological history

Formed through the rapid cooling of felsic (silica-rich) lava, preventing crystal growth. Most specimens are geologically young, typically less than 20 million years old (Tertiary to Quaternary), because the glass eventually devitrifies into crystals over time.

Uses & applications

Historically used for stone tools and weaponry (arrowheads, knives). Currently used for surgical scalpels, lapidary art/jewelry, and as an ornamental stone in home decor.

Geological facts

Obsidian fractures with an edge thinner and sharper than a steel scalpel, down to a nearly molecular level. The visible banding in this specimen is known as flow banding, caused by the stretching of bubbles or inclusion layers within the lava flow.

Field identification & locations

Identify by its glass-like texture, sharp edges, and unique curved 'conchoidal' fracture patterns. Common in volcanic regions with high silica rhyolitic eruptions, such as the Western United States (Oregon, California), Mexico, and Iceland. Collectors look for smooth texture and clear banding.

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

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