igneous

Basalt (Road Aggregate)

Basalt (Aphanitic Mafic Igneous Rock)

Hardness: 6 (Mohs scale); Color: Dark gray to charcoal black; Luster: Dull/Earthy; Crystal structure: Fine-grained/Aphanitic; Specific gravity: 2.8 - 3.0. This specimen appears weathered/rounded with a slight sheen.

Hardness: 6 (Mohs scale)
Color: Dark gray to charcoal black
Luster: Dull/Earthy

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 (extrusive). Most common in oceanic crust and volcanic regions.

Uses & applications

Predominantly used as crushed stone aggregate for road construction, railroad ballast, and concrete production due to its durability and compressive strength.

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 found in abundance on the Moon and Mars (the dark 'seas' or Maria on the Moon are basaltic plains).

Field identification & locations

Identify by its dark color, heavy weight (density), and lack of visible crystals to the naked eye. Commonly found in gravel driveways, near railroad tracks, or in areas with volcanic history.

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

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral