igneous

Obsidian

Obsidian (Volcanic Glass, primarily SiO2)

Hardness: 5–5.5 (Mohs scale); Color: Black to dark grey; Luster: Vitreous/Glassy; Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.35–2.6.

Hardness: 5–5
Color: Black to dark grey
Luster: Vitreous/Glassy

Identified March 26, 2026More igneous →

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Physical properties

Hardness: 5–5.5 (Mohs scale); Color: Black to dark grey; Luster: Vitreous/Glassy; Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.35–2.6.

Formation & geological history

Formed as an extrusive igneous rock when felsic lava (rich in silica) cools so rapidly that crystals do not have time to grow. Most specimens are geologically young, typically from the Cenozoic era, as glass eventually devitrifies into minerals over millions of years.

Uses & applications

Historically used for making sharp tools like arrowheads and knives. Modern uses include surgical scalpel blades, jewelry (cabochons), and ornamental carvings.

Geological facts

Because of its lack of crystal structure, obsidian breaks into pieces with edges thinner and sharper than high-quality steel scalpels. It was highly prized in Mesoamerican cultures for both weaponry and ritual mirrors.

Field identification & locations

Identify in the field by its distinct conchoidal (shell-like) fracture patterns and sharp, translucent edges. It is commonly found in volcanic regions such as the Western United States, Mexico, Iceland, and Italy. Collectors should look for smoothness and the absence of visible grains.

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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