igneous

Basalt (Water-worn Pebble)

Basalt (Aphanitic Mafic Igneous Rock)

Hardness: 6 (Mohs scale); Color: Dark grey to black, sometimes with greenish tints; Luster: Dull to sub-vitreous when wet; Crystal structure: Fine-grained/Aphanitic; Cleavage: None; Specific gravity: 2.8–3.0

Hardness: 6 (Mohs scale)
Color: Dark grey to black, sometimes with greenish tints
Luster: Dull to sub-vitreous when wet

Identified March 28, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. This specific specimen has been smoothed and rounded by river or waves (fluvial/marine erosion) over hundreds to thousands of years.

Uses & applications

Commonly used as crushed stone in construction, road base, and railroad ballast. Smooth stones like this are often used in landscaping, decorative 'river rock' arrangements, and occasionally for hot stone massage therapy.

Geological facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also extremely common on the Moon and Mars. Many basaltic formations on Earth date back millions of years to volcanic events.

Field identification & locations

Identify by its dark, heavy feel and lack of visible large crystals to the naked eye. In the field, look for it in volcanic regions or as rounded pebbles in streambeds and ocean beaches. It will not scratch easily with a copper penny.

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