Igneous

Obsidian

Volcanic Glass (SiO2 rich)

Hardness: 5-5.5 (Mohs scale); Color: Typically deep black (sometimes greenish, mahogany, or rainbow); Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like); Specific Gravity: 2.3-2.6

Hardness: 5-5
Luster: Vitreous (glassy)

Identified March 5, 2026More igneous →

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Physical properties

Formation & geological history

Formed from extrusive volcanic activity when highly viscous, silica-rich lava cools extremely rapidly, preventing the growth of mineral crystals. It is geologically young as it eventually devitrifies into fine-grained minerals.

Uses & applications

Historically used for weapons (arrowheads, knives) due to its razor-sharp edges. Currently used in specialized medical scalpels, jewelry (beads, cabochons), ornamental carvings, and for meditation/metaphysical collecting.

Geological facts

Obsidian can be fractured to reveal an edge that is much thinner and sharper than a high-quality steel scalpel. Because it lacks crystal structure, it is technically a 'mineraloid' rather than a true mineral.

Field identification & locations

Identify in the field by its distinct conchoidal fracture (curved, glass-like breaks) and lack of visible grains. Commonly found in areas with geologically recent volcanic activity, such as the Western USA (Oregon, California), Mexico, and Iceland.

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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