Igneous

Basalt

Mafic Volcanic Rock (primarily Plagioclase and Pyroxene)

Hardness: 6.0 (Mohs scale); Color: Dark gray to black; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific gravity: 2.8–3.0

Hardness: 6
Color: Dark gray to black
Luster: Dull to sub-vitreous

Identified March 18, 2026More igneous →

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Physical properties

Hardness: 6.0 (Mohs scale); Color: Dark gray to black; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific gravity: 2.8–3.0

Formation & geological history

Formed from the rapid cooling of low-viscosity lava rich in magnesium and iron. They occur at divergent boundaries, oceanic hotspots, and mantle plumes. Most basalt currently on Earth is from the Cenozoic era, though it has formed throughout geological history.

Uses & applications

Extensively used in construction as crushed stone for road base, railroad ballast, and concrete aggregate. It is also used for building blocks, floor tiles, and in industrial 'cast basalt' for high-abrasion applications. Fine specimens are used in landscaping.

Geological facts

Basalt is the most common rock type in Earth's crust, covering most of the ocean floors. It is also found on the Moon (the dark lunar maria are basaltic) and Mars. It can form spectacular columnar jointing as it cools and shrinks.

Field identification & locations

Identify in the field by its heavy weight (dense), dark color, and fine-grained texture. It often shows small circular vesicles (air bubbles) or tiny white/green crystals. Common globally, especially in volcanic regions like Iceland, Hawaii, and the Pacific Northwest.

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