igneous

Basalt River Pebble

Aphanitic Mafic Basalt

Hardness: 6 (Mohs scale); Color: Dark grey to greenish-black; Luster: Dull or earthy (matte); Crystal Structure: Aphanitic (fine-grained); Cleavage: None; Specific Gravity: 2.8-3.0.

Hardness: 6 (Mohs scale)
Color: Dark grey to greenish-black
Luster: Dull or earthy (matte)

Identified March 28, 2026More igneous →

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Physical properties

Hardness: 6 (Mohs scale); Color: Dark grey to greenish-black; Luster: Dull or earthy (matte); Crystal Structure: Aphanitic (fine-grained); Cleavage: None; 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 and smoothed through fluvial erosion (water transport) in a river environment over thousands of years.

Uses & applications

Used primarily in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Small polished pebbles are used in landscaping, aquariums, and decorative stone mulch.

Geological facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floors. It also exists on the Moon (the dark 'maria' plains) and Mars.

Field identification & locations

Identify by its dark color, heavy weight for its size, and lack of visible crystals to the naked eye. Commonly found in riverbeds near volcanic regions or tectonic plate boundaries. Collectors should look for unique shapes or smooth textures; this specimen shows characteristic 'river-wear'.

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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