Igneous

Obsidian

Obsidian (Volcanic Glass), primarily SiO2 (70% or more silicon dioxide)

Hardness: 5–5.5 Mohs; Color: Typically black but can be brown or green; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None, exhibits conchoidal fracture; Specific Gravity: 2.35–2.6

Hardness: 5–5
Color: Typically black but can be brown or green
Luster: Vitreous (glassy)

Identified March 15, 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 rapidly cooling felsic lava where the cooling happens so quickly that crystals do not have time to grow. Most specimens are geologically young (less than few million years old) because it eventually devitrifies into crystalline rock.

Uses & applications

Historically used for razor-sharp tools and weapons. Today used in scalpel blades for delicate surgery, modern jewelry, and decorative items due to its aesthetic appeal.

Geological facts

Obsidian fractures with an edge that is many times sharper than a high-quality steel scalpel. Ancient Aztecs used an obsidian-bladed sword called a Macuahuitl.

Field identification & locations

Identify in the field by its distinct conchoidal (shell-like) fracture, intense glassy luster, and sharp edges when broken. Commonly found in volcanic regions such as the American Northwest, Mexico, and Iceland.

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