igneous

Basalt (Crushed Stone Aggregate)

Mafic Volcanic Basalt (Major Minerals: Plagioclase Feldspar, Pyroxene, Olivine)

Hardness: 5-6 (Mohs scale); Color: Dark grey to black; Luster: Dull to earthy; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0.

Hardness: 5-6 (Mohs scale)
Color: Dark grey to black
Luster: Dull to earthy

Identified March 10, 2026More igneous →

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Physical properties

Hardness: 5-6 (Mohs scale); Color: Dark grey to black; Luster: Dull to earthy; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0.

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at the Earth's surface. This specimen appears to be an aggregate embedded in concrete, which is common in modern infrastructure.

Uses & applications

Primary use as road base, railroad ballast, and concrete aggregate in construction over millions of tons annually.

Geological facts

Basalt is the most common rock type in Earth's crust and makes up most of the ocean floor. It is also found extensively on the Moon and Mars.

Field identification & locations

Identify in the field by its very dark color, heavy weight compared to other rocks of similar size, and lack of visible crystals to the naked eye. Often found in volcanic regions or as imported construction material.

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