igneous

Basalt (Vesicular)

Extrusive mafic igneous rock; primarily (Mg, Fe)2SiO4 (Olivine), CaAl2Si2O8 (Plagioclase Feldspar), and Pyroxene

Hardness: 6 (Mohs scale); Color: Dark grey to coal-black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; Specific gravity: 2.8–3.0

Hardness: 6 (Mohs scale)
Color: Dark grey to coal-black
Luster: Dull to sub-metallic

Identified March 21, 2026More igneous →

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Physical properties

Hardness: 6 (Mohs scale); Color: Dark grey to coal-black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; 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 moon. Vesicular varieties form when gas bubbles are trapped in the cooling lava. Found in volcanic regions ranging from the Cenozoic to the Archean eras.

Uses & applications

Extensively used in construction as aggregate for concrete and asphalt, railroad ballast, and high-quality paving stones. It is also used as raw material for rock wool (an insulating material).

Geological facts

Basalt is the most common rock type in the Earth's crust, making up most of the ocean floor. It is also found in abundance on the Moon and Mars. Many famous landmarks, such as the Giant's Causeway in Ireland, are comprised of basalt columns.

Field identification & locations

Identify in the field by its high density, dark color, and fine texture. Vesicular basalt is distinctive for its small holes (vesicles). It is commonly found in volcanic fields, ocean ridges, and flood basalt plateaus. Collectors look for specimens containing olivine (green) or quartz crystals.

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