igneous

Basalt

Basalt (Aphanitic-fine grained mafic igneous rock)

Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull to sub-metallic; Crystal structure: Microcrystalline (aphanitic), typically consisting of plagioclase feldspar and pyroxene; Cleavage: None (conchoidal fracture); Specific gravity: 2.8–3.0.

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

Identified March 25, 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 a moon. Most basalt on Earth is formed at mid-ocean ridges. It can range from modern flows to billions of years old.

Uses & applications

Commonly used in construction for aggregate in concrete and asphalt, road base, and railroad ballast. In jewelry, it is occasionally used as 'lava stone' beads. In industry, it is melted to create basalt fiber and stone wool insulation.

Geological facts

Basalt is the most common rock type in the Earth's crust and covers most of the ocean floor. It is also the primary volcanic rock on Mars and the Moon (the lunar 'maria' or dark seas are basaltic plains).

Field identification & locations

Identify in the field by its dark color, heavy weight (relative to size due to high iron content), and very fine crystalline grain. It is found globally in volcanic regions such as Iceland, Hawaii, and the Pacific Northwest. Collectors look for vesicular basalt (containing gas bubbles) or specimens with olivine inclusions.

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