igneous

Basalt Pebble

Aphanitic Mafic Igneous Rock

Hardness: 6-7 (Mohs scale), Color: Dark grey to black, Luster: Dull/Earthy, Crystal Structure: Aphanitic (fine-grained), Cleavage: None (conchoidal fracture), Specific Gravity: 2.8-3.0

Identified March 17, 2026More igneous →

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Physical properties

Hardness: 6-7 (Mohs scale), Color: Dark grey to black, Luster: Dull/Earthy, Crystal Structure: Aphanitic (fine-grained), Cleavage: None (conchoidal fracture), 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 Earth's surface. This specific specimen is a 'river rock' or 'beach pebble', having been rounded by water erosion over hundreds to thousands of years.

Uses & applications

Used extensively in construction as aggregate, for road base, and railroad ballast. Smooth pebbles are often used in decorative landscaping and 'hot stone' massage therapy.

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.

Field identification & locations

Identified by its dark color, heavy weight for its size, and absence of visible large crystals. Common in volcanic regions, riverbeds, and coastal areas. Collectors look for unique shapes or vesicles.

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