igneous

Obsidian

Obsidian (Volcanic Glass), primarily SiO2 (Silicon Dioxide)

Hardness: 5–5.5 on Mohs scale; Color: Black, deep brown, or greyish with occasional bands; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35–2.6

Hardness: 5–5
Color: Black, deep brown, or greyish with occasional bands
Luster: Vitreous (glassy)

Identified March 24, 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 from the rapid cooling of silica-rich (rhyolitic) lava, which cools so quickly that mineral crystals do not have time to grow. Most specimens are geologically young (less than few million years old) because glass is unstable and devitrifies over time.

Uses & applications

Historically used for making sharp tools like arrowheads and knives; currently used in jewelry (cabochons), scalpel blades for delicate surgeries, and as a decorative stone in landscaping and collection.

Geological facts

Obsidian is often called 'nature's glass'. Despite its dark appearance, thin slivers are often translucent. Some varieties like 'Rainbow' or 'Sheen' obsidian contain microscopic mineral inclusions that create colorful iridescence.

Field identification & locations

Identify in the field by its distinct conchoidal (shell-like) fracture patterns and sharp edges. Common in volcanic regions such as the American West (Oregon, California), Mexico, Iceland, and Japan. Collectors should look for smoothness and lack of internal bubbles found in slag glass.

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

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

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

mineral

Magnetite

Magnetite (Fe3O4)

mineral