igneous

Basalt

Mafic Igneous Rock (primarily Plagioclase and Pyroxene)

Hardness: 6 (Mohs), Color: Dark grey to black, Luster: Dull/Sub-metallic, Crystal structure: Fine-grained/Aphanitic, Cleavage: None (conchoidal fracture), Specific Gravity: 2.8–3.0

Identified March 29, 2026More igneous →

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Physical properties

Hardness: 6 (Mohs), Color: Dark grey to black, Luster: Dull/Sub-metallic, Crystal structure: Fine-grained/Aphanitic, 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 at or very near the Earth's surface. Found in oceanic crust and volcanic regions; ages vary from modern to billions of years old.

Uses & applications

Used extensively in construction as aggregate for roads, railroad ballast, and concrete. It is also used as a thermal insulator (rock wool) and for high-durability floor tiles.

Geological facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. Large-scale cooling can result in 'Columnar Basalt' formations like the Giant's Causeway.

Field identification & locations

Identify in the field by its high density, dark color, and lack of visible crystals. It will not scratch easily with steel. Common in volcanic zones and coastal regions.

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