igneous

Basalt

Mafic volcanic rock (formula varies by mineral content: Ca-plagioclase, Pyroxene, Olivine)

Hardness: 6 on Mohs scale; Color: dark grey to black; Luster: dull to earthy; Crystal structure: aphanitic (fine-grained); Cleavage: none; Specific gravity: 2.8–3.0.

Hardness: 6 on Mohs scale
Color: dark grey to black
Luster: dull to earthy

Identified April 5, 2026More igneous →

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Physical properties

Hardness: 6 on Mohs scale; Color: dark grey to black; Luster: dull to earthy; Crystal structure: aphanitic (fine-grained); Cleavage: none; 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 characterizes much of Earth's oceanic crust.

Uses & applications

Extensively used in construction for aggregate, road base, concrete, and railroad ballast. Also used as dimension stone and in high-performance fiber production (basalt fiber).

Geological facts

Basalt is the most common rock type in the Earth's crust and also makes up most of the surface of the Moon and Mars. On Earth, it forms the iconic 'Giant's Causeway' in Ireland through columnar jointing.

Field identification & locations

Identify by its dark color, heavy weight (density), and fine-grained texture where individual crystals are too small to see with the naked eye. Commonly found in volcanic fields and oceanic ridges.

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

Blue Apatite

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

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral