igneous

Basalt with Vesicles

Aphanitic Mafic Volcanic Rock (Basalt)

Hardness: 6 (Mohs scale); Color: Dark grey to brownish-olive (weathered surface); Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained, invisible to the naked eye); Specific gravity: 2.8–3.0

Hardness: 6 (Mohs scale)
Color: Dark grey to brownish-olive (weathered surface)
Luster: Dull/Earthy

Identified March 27, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at the earth's surface. Small pits (vesicles) occur where gas bubbles were trapped as the lava solidified. Common in regions of oceanic crust and volcanic hotspots.

Uses & applications

Commonly used in construction as aggregate for concrete and asphalt, railroad ballast, and occasionally for high-durability floor tiles or garden landscaping.

Geological facts

Basalt is the most common rock type in the Earth's crust, making up most of the ocean floor; it is also highly common on the Moon and Mars.

Field identification & locations

Identify in the field by its high density relative to other dark stones, its fine-grained texture, and the presence of small rounded pits or gas bubbles. Commonly found on beaches or in riverbeds where it has been smoothed into a cobble/pebble shape.

Other igneous specimens

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

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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

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Blue Apatite

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

mineral

Magnetite

Magnetite (Fe3O4)

mineral