igneous

Obsidian / Volcanic Glass

Obsidian (principally SiO2, usually 70% or more)

Hardness: 5–5.5 Mohs scale; Color: typically black but can be dark green or brown; Luster: Vitreous (glassy); Crystal Structure: Amorphous (lacks crystal structure due to rapid cooling); Cleavage: Conchoidal fracture (shell-like curves); Specific Gravity: 2.35–2.6

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

Identified March 18, 2026More igneous →

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Physical properties

Formation & geological history

Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Common in geologically young areas of volcanic activity (Cenozoic era).

Uses & applications

Historically used for edged tools and weapons (arrowheads, knives); modern use in surgical scalpel blades, jewelry (cabochons/beads), and ornamental carvings.

Geological facts

Because it is a glass and not a crystal, it is metastable and will eventually weather into perlite through hydration over millions of years. It can produce an edge many times sharper than high-quality steel surgical scalpels.

Field identification & locations

Identify by its very smooth, glassy texture and distinctive razor-sharp conchoidal fracture patterns. Commonly found in volcanic fields like the American Southwest, Iceland, Mexico, and Japan. For collectors: look for smooth curved edges when broken.

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

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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

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Blue Apatite

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

mineral

Magnetite

Magnetite (Fe3O4)

mineral