igneous

Basalt (River-worn Pebble)

Aphanitic Mafic Igneous Rock

Hardness: 6 on Mohs scale; Color: Dark grey to dull black; Luster: Dull/Sub-metallic (when wet); Crystal structure: Aphanitic (fine-grained, crystals not visible to eye); Cleavage: None/Conchoidal fracture; Specific gravity: 2.8-3.0.

Hardness: 6 on Mohs scale
Color: Dark grey to dull black
Luster: Dull/Sub-metallic (when wet)

Identified May 26, 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 chemically weathered and physically rounded by fluvial (river) or marine transport.

Uses & applications

Basalt is used extensively in construction as aggregate for roads, railroad ballast, and in concrete. Polished pebbles are often used in landscaping, decorative stone displays, 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. Large basaltic flows are known as 'traps'.

Field identification & locations

Identify in the field by its dark color, heavy weight for its size (high density), and lack of visible crystals. It often develops a thin, lighter-colored weathering rind. Commonly found near volcanic sites or in riverbeds draining volcanic regions.

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

Dalmatian Jasper

Aplite with Arfvedsonite inclusions

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