igneous

Basalt (River Pebble)

Basalt (Aphanitic Mafic Volcanic Rock)

Hardness: 6-7 (Mohs); Color: Dark grey to charcoal black; Luster: Dull to slightly waxy when wet; Crystal structure: Aphanitic (fine-grained, microscopic crystals); Specific gravity: 2.8–3.0.

Hardness: 6-7 (Mohs)
Color: Dark grey to charcoal black
Luster: Dull to slightly waxy when wet

Identified March 16, 2026More igneous →

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Physical properties

Hardness: 6-7 (Mohs); Color: Dark grey to charcoal black; Luster: Dull to slightly waxy when wet; Crystal structure: Aphanitic (fine-grained, microscopic crystals); Specific gravity: 2.8–3.0.

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 long-term hydraulic action in a river or coastal environment.

Uses & applications

Used extensively in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Smooth river-worn basalt is popular for decorative landscaping, aquariums, and 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 found on the Moon, Mars, and Venus.

Field identification & locations

Identify in the field by its high density (feels heavy for its size), dark color, and lack of visible large crystals. Most common in volcanic regions or transported downstream in riverbeds.

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