Igneous

Basalt

Mafic Volcanic Rock (Primarily Plagioclase and Pyroxene)

Hardness: 5-6 (Mohs scale); Color: Dark gray to black, often weathering to brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained/microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.8–3.0.

Hardness: 5-6 (Mohs scale)
Color: Dark gray to black, often weathering to brown
Luster: Dull to sub-metallic

Identified March 16, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. Most specimens are from the Cenozoic era, though it has formed throughout Earth's history via volcanic activity and seafloor spreading.

Uses & applications

Used extensively in construction as crushed stone for road base, concrete aggregate, and railroad ballast. In industry, it is melted to create stone wool insulation or cast into floor tiles. Fine-grained pieces with high density are also used in shoreline protection (riprap).

Geological facts

Basalt is the most common rock type in Earth's crust; it makes up most of the ocean floors. It is also found on other planetary bodies like the Moon (lunar maria), Mars, and Venus. Large formations, like the Giant's Causeway, showcase its ability to form hexagonal columns during cooling.

Field identification & locations

Identify in the field by its dark color, heavy weight (relative to size), and very fine grains that are difficult to see without a lens. Look for small vesicles (gas bubbles) or tiny green olivine crystals. Common in volcanic regions like Hawaii, the Pacific Northwest, and Iceland.

Other igneous specimens

See all

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