igneous

Basalt

Basalt (Fine-grained extrusive igneous rock)

Hardness: 6 (Mohs scale); Color: Dark grey to black, sometimes weathering to brown or green; Luster: Dull or sub-metallic; Crystal Structure: Aphanitic (fine-grained, crystals not visible to eye); Specific Gravity: 2.8–3.0.

Hardness: 6 (Mohs scale)
Color: Dark grey to black, sometimes weathering to brown or green
Luster: Dull or sub-metallic

Identified March 29, 2026More igneous →

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Physical properties

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 moon. It is the most common rock type in the Earth's crust, forming most of the ocean floor and large volcanic plateaus.

Uses & applications

Extensively used in construction for aggregate (road base, railroad ballast), paving stones, high-strength concrete, and as a raw material for 'stone wool' insulation. Fine specimens are sometimes used for aquarium landscaping.

Geological facts

Basalt makes up the majority of the ocean floor and is the most common rock type on the Moon. Famous formations like the Giant's Causeway in Northern Ireland showcase basalt's tendency to cool into hexagonal 'columns'.

Field identification & locations

Identify in the field by its dark color, heavy weight (density), and fine-grained texture that lacks visible quartz. Often found near volcanic regions, rift valleys, or oceanic crust outcrops. It lacks the 'holes' of scoria but may have small vesicles.

Other igneous specimens

See all

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