igneous

Basalt (Water-worn Pavement Stone)

Basalt (Mafic Extrusive Igneous Rock)

Hardness: 6 (Mohs scale); Color: dark grey to black (appears darker when wet or polished); Luster: dull to sub-glassy; Crystal structure: fine-grained (aphanitic) or porphyritic; Cleavage: none (conchoidal fracture); Specific gravity: 2.8 - 3.0.

Hardness: 6 (Mohs scale)
Color: dark grey to black (appears darker when wet or polished)
Luster: dull to sub-glassy

Identified March 28, 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 surface of a terrestrial planet or a moon. This specimen appears water-worn, smoothed by river or beach action. Its geological age can range from recent Cenozoic to over 3.5 billion years old.

Uses & applications

Used primarily in construction for aggregate, road base, and railroad ballast. Exceptionally smooth or dark specimens are used in stone massage therapy, as garden decor, or as aquarium substrate.

Geological facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also common on the Moon (the dark lunar maria) and Mars.

Field identification & locations

Identify in the field by its weight (heavy for its size), dark uniform color, and fine-grained texture with no visible individual crystals unless looking through a lens. Commonly found near volcanic sites, coastlines, and riverbeds.

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