igneous

Basalt (River Stone)

Basalt (Extrusive Igneous Rock)

Hardness: 6-7 Mohs; Color: Dark grey to black, often with greenish or brownish weathering; Luster: Dull to sub-metallic (but shiny if wet or polished); Crystal Structure: Aphanitic (fine-grained); Specific Gravity: 2.8–3.0

Hardness: 6-7 Mohs
Luster: Dull to sub-metallic (but shiny if wet or polished)

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 exposed at or very near the surface of a terrestrial planet or a moon. This specimen shows a 'river cobble' morphology, meaning it was shaped by mechanical erosion in water over thousands of years.

Uses & applications

Used primarily in construction for crushed stone, road bases, and concrete aggregate. Smooth river basalt is popular in landscaping, aquarium decor, and hot stone massage therapy.

Geological facts

Basalt is the most common rock type in the Earth\u0027s crust and makes up most of the ocean floor. It is also found extensively on the Moon and Mars.

Field identification & locations

Identify in the field by its weight (denser than most light rocks), dark color, and fine-grained texture. To identify a river stone, look for the characteristic rounded, smooth edges caused by water tumbling.

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