igneous

Basalt River Cobble

Basalt (Aphanitic Mafic Volcanic Rock)

Hardness: 6-7 (Mohs scale); Color: Dark grey to charcoal, weathering to olive-brown; Luster: Dull/Earthy; Structure: Fine-grained (aphanitic) with a smooth, water-worn texture; Cleavage: None; Specific Gravity: 2.8-3.0.

Hardness: 6-7 (Mohs scale)
Color: Dark grey to charcoal, weathering to olive-brown
Luster: Dull/Earthy

Identified March 22, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of low-viscosity magnesium and iron-rich lava. This specific specimen has been significantly rounded and smoothed by fluvial (river) erosion over thousands of years.

Uses & applications

Commonly used in landscaping as decorative 'river rock,' as crushed stone for road construction base, and in the production of high-strength insulation (mineral wool).

Geological facts

Basalt is the most common rock type in the Earth's crust and covers most of the ocean floor. It is also found extensively on the Moon and Mars.

Field identification & locations

Identify by its dark color, heavy weight for its size, and smooth, rounded shape if found in riverbeds. It is extremely common in the Pacific Northwest and volcanic regions. Collectors look for pieces with interesting mineral inclusions or vesicular 'gas bubble' holes.

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