igneous

Basalt

Mafic volcanic rock (primarily plagioclase feldspar and pyroxene)

Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull/Sub-metallic; 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
Luster: Dull/Sub-metallic

Identified February 27, 2026More igneous →

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Physical properties

Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull/Sub-metallic; Structure: Fine-grained (aphanitic) or porphyritic; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8–3.0

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. It is the most common volcanic rock on Earth, often found in oceanic crust and trap rock formations.

Uses & applications

Used primarily in construction for road base, concrete aggregate, and railroad ballast. It is also used in the manufacturing of rock wool insulation and as polished dimension stone for floor tiles and monuments.

Geological facts

Basalt is the primary rock type found on the Moon (the Lunar Maria are basaltic) and on Mars. It forms unique hexagonal columns when cooling, such as the Giant's Causeway in Northern Ireland.

Field identification & locations

Identify in the field by its very dark color, heavy weight (density), and lack of visible large crystals. It often feels slightly gritty and exhibits sharp edges when broken. Common globally, especially in volcanic regions like Hawaii or the Pacific Northwest.

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