igneous

Basalt

Basalt (SiO2 percentage between 45% and 52%)

Hardness: 6 on Mohs scale; Color: Dark grey to black, often weathering to brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained), often contains micro-crystals of plagioclase feldspar and pyroxene; Cleavage: None (conchoidal fracture); Specific gravity: 2.8-3.0.

Hardness: 6 on Mohs scale
Color: Dark grey to black, often weathering to brown
Luster: Dull to sub-metallic

Identified March 26, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or moon. Most Basalt specimens range from the Archean Eon to recent volcanic eruptions.

Uses & applications

Used primarily in construction for aggregate, road base, railroad ballast, and floor tiles. It is also used as cast basalt for industrial wear-resistant liners.

Geological facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It also covers large areas of the Moon (the Lunar Maria), Mars, and Venus.

Field identification & locations

Identify in the field by its dark weight and fine-grained texture; it may have small holes (vesicles) if gas bubbles were trapped during cooling. Found globally in volcanic regions like the Pacific Northwest of the USA, Iceland, and Hawaii.

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