Rock Identifier

igneous

Basalt (Road Aggregate / Crushed Stone)

Aphanitic Mafic Igneous Rock

Hardness: 6 (Mohs scale); Color: Dark grey to charcoal/black; Luster: Dull to sub-metallic when fresh; Crystal structure: Fine-grained/Aphanitic (not visible to naked eye); Cleavage: None (conchoidal fracture); Specific gravity: 2.8–3.0.

Hardness: 6 (Mohs scale)
Color: Dark grey to charcoal/black
Luster: Dull to sub-metallic when fresh

Identified March 22, 2026More igneous →

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Physical properties

Hardness: 6 (Mohs scale); Color: Dark grey to charcoal/black; Luster: Dull to sub-metallic when fresh; Crystal structure: Fine-grained/Aphanitic (not visible to naked eye); Cleavage: None (conchoidal fracture); Specific gravity: 2.8–3.0.

Formation & geological history

Formed from the rapid cooling of low-viscosity lava rich in magnesium and iron at or very near the Earth's surface. Found in volcanic provinces and oceanic tectonic plates.

Uses & applications

Used primarily as road base, railroad ballast, and concrete aggregate due to its high compressive strength and durability. Also used for pavers and floor tiles.

Geological facts

Basalt is the most common rock type in the Earth's crust, making up most of the ocean floor. It also exists on the Moon and Mars in the form of 'Maria' (seas).

Field identification & locations

Identify in the field by its dark color, heavy weight (density), and fine-grained texture. Found abundantly in the Pacific Northwest, Hawaii, and Iceland.

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Blue Apatite (Fluorapatite (Ca5(PO4)3F)) — mineral

Blue Apatite

Fluorapatite (Ca5(PO4)3F)

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

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.

Magnetite (Iron(II,III) oxide (Fe3O4)) — mineral

Magnetite

Iron(II,III) oxide (Fe3O4)

Blue Apatite (Apatite (Group), Ca5(PO4)3(F,Cl,OH)) — mineral

Blue Apatite

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

Sodalite (Sodalite - Na8(Al6Si6O24)Cl2) — mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

Magnetite (Magnetite (Fe3O4)) — mineral

Magnetite

Magnetite (Fe3O4)