igneous

River Rock (Basalt)

Basalt (SiO2 range: 45-52%)

Hardness: 6 on Mohs scale; Color: typically dark gray to black, often weathered to brown; Luster: dull to vitreous (especially when wet); Crystal structure: aphanitic (fine-grained); Cleavage: none; Specific gravity: 2.8-3.0.

Hardness: 6 on Mohs scale
Color: typically dark gray to black, often weathered to brown
Luster: dull to vitreous (especially when wet)

Identified March 18, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at the Earth's surface. Extrusive igneous rock. Ages vary widely by location (from recent to hundreds of millions of years). The rounded shape is the result of mechanical weathering and abrasion within a high-energy aquatic environment, such as a riverbed or shoreline.

Uses & applications

Used extensively in construction (aggregate, road base), landscaping, and for decorative use in home aquariums or Zen gardens. Sometimes used for therapeutic 'hot stone' massages.

Geological facts

Basalt is the most common volcanic rock on Earth and also makes up most of the ocean floors. It has been found on the Moon and Mars as well. The dark, flat plains seen on the Moon (Maria) are basaltic lava flows.

Field identification & locations

Identify in the field by its dark, uniform color, heavy weight for its size, and smooth, rounded water-worn texture. Commonly found in riverbeds, glacial deposits, and along coastal beaches in volcanic regions.

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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