igneous

Basalt (Road Gravel)

Aphanitic mafic volcanic rock (various compositions of plagioclase feldspar and pyroxene)

Hardness: 6 (Mohs scale); Color: Dark grey to black, weathering to brown/tan; Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific Gravity: 2.8–3.0

Hardness: 6 (Mohs scale)
Color: Dark grey to black, weathering to brown/tan
Luster: Dull/Earthy

Identified March 28, 2026More igneous →

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Physical properties

Hardness: 6 (Mohs scale); Color: Dark grey to black, weathering to brown/tan; Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific Gravity: 2.8–3.0

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. These specimens are often sourced from volcanic fields of various geological ages, often Cenozoic to recent for modern construction use.

Uses & applications

Primarily used as construction aggregate, road base, railroad ballast, and in asphalt/concrete mixtures. Larger pieces can be used for landscaping.

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 extremely common on the Moon and Mars.

Field identification & locations

Identify in the field by its dark color, heavy weight (density), and fine-grained texture with no visible crystals to the naked eye. Often found in driveways and alongside roads as crushed stone.

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