igneous

Basalt River Cobble

Extrusive Mafic Igneous Rock (primarily Plagioclase and Pyroxene)

Hardness: 6-7 on Mohs scale; Color: Dark grey to charcoal; Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8-3.0.

Hardness: 6-7 on Mohs scale
Color: Dark grey to charcoal
Luster: Dull/Earthy

Identified May 26, 2026More igneous →

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Physical properties

Hardness: 6-7 on Mohs scale; Color: Dark grey to charcoal; Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained); 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 specimen has been rounded by fluvial (river) erosion over thousands of years.

Uses & applications

Commonly used in construction aggregate, road base, and as decorative landscaping stones. Smooth river cobbles are also popular for hot stone massage therapy and decorative garden features.

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 dark color, heavy weight (relative to size), and lack of visible crystals. It is ubiquitous in volcanic regions and riverbeds downstream from volcanic mountain ranges.

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

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