igneous

Obsidian

Volcanic glass (mainly SiO2 with inclusions)

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

Hardness: 5–5
Color: Black, deep grey, or dark brown
Luster: Vitreous (glassy)

Identified March 25, 2026More igneous →

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Physical properties

Formation & geological history

Formed from extrusive igneous activity when felsic lava cools rapidly with minimal crystal growth. Common in rhyolitic volcanic eruptions spanning from the Cenozoic era to recent times.

Uses & applications

Historically used for spearheads and blades; currently used in jewelry (cabochons/faceting), ornamental carvings, and high-precision surgical scalpels.

Geological facts

Because it is essentially a supercooled liquid, obsidian is metastable and eventually devitrifies into quartz and feldspar over millions of years. It lacks the crystalline regularity of minerals, making it a mineraloid.

Field identification & locations

Identify in the field by its sharp, curved (conchoidal) edges and glass-like appearance. Commonly found in volcanic regions such as the American Northwest (Oregon, California), Mexico, Iceland, and Italy (Lipari).

Other igneous specimens

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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