igneous

Basalt River Rock

Basalt (Aphanitic Mafic Igneous Rock)

Hardness: 6-7 on Mohs scale; Color: Dark grey to black; Luster: Dull/Earthy; Crystal structure: Fine-grained/Aphanitic (minerals too small to see with the naked eye); Cleavage: None; Specific Gravity: 2.8–3.0.

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

Identified March 22, 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 and mechanically weathered by water (fluvial processes), resulting in its smooth, rounded 'river rock' shape.

Uses & applications

Used extensively in construction as crushed stone (aggregate) for roads and concrete. Smooth, rounded specimens are popular in landscaping, xeriscaping, and massage therapy (hot stone massage).

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 and Mars. Most basaltic lava on Earth is less than 200 million years old.

Field identification & locations

Identify by its dark color, heavy weight (high density), and fine-grained texture. It will not scratch easily with a steel blade. Found globally in volcanic regions, riverbeds, and as common landscaping gravel.

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