igneous

Basalt (River Pebble)

Extrusive mafic igneous rock

Hardness: 6-7 on Mohs scale; Color: Dark grey to black; Luster: Dull to sub-metallic when dry, waxy when wet; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8-3.0

Hardness: 6-7 on Mohs scale
Color: Dark grey to black
Luster: Dull to sub-metallic when dry, waxy when wet

Identified April 9, 2026More igneous →

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Physical properties

Hardness: 6-7 on Mohs scale; Color: Dark grey to black; Luster: Dull to sub-metallic when dry, waxy when wet; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8-3.0

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the surface of a planet. This specific specimen has been rounded by fluvial (river) or glacial erosion over thousands of years.

Uses & applications

Used extensively in construction as aggregate (crushed stone), for railroad ballast, and in the manufacturing of stone wool insulation. Smooth river basalt is popular in landscaping and massage therapy (hot stones).

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 extremely common on the Moon and Mars.

Field identification & locations

Identify in the field by its dark color, heavy weight (high density), and fine-grained texture. It will not react with acid. Commonly found in volcanic regions, riverbeds, and beaches.

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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