Igneous

Basalt (Polished River Stone)

Mafic Volcanic Rock (primarily Plagioclase and Pyroxene)

Hardness: 5-6 (Mohs); Color: Dark grey to black; Luster: Dull to sub-vitreous; Structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None; Specific Gravity: 2.8–3.0

Hardness: 5-6 (Mohs)
Color: Dark grey to black
Luster: Dull to sub-vitreous

Identified March 23, 2026More igneous →

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Physical properties

Hardness: 5-6 (Mohs); Color: Dark grey to black; Luster: Dull to sub-vitreous; Structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None; Specific Gravity: 2.8–3.0

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. This specimen shows signs of tumbling/polishing, likely from high-energy water environments like rivers or beaches.

Uses & applications

Commonly used in construction (aggregate), as a decorative landscaping stone, in hot stone massage therapies, and for aquarium decor.

Geological facts

Basalt is the most common rock type in the Earth's crust, making 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 for its size, and lack of visible crystals without a lens. Often found in volcanic regions or as rounded cobbles 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