igneous

Basalt (tumbled/river-worn)

Mafic volcanic rock (primarily plagioclase feldspar and pyroxene)

Hardness: 5-6 (Mohs); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: Non-existent; Specific Gravity: 2.8–3.0

Hardness: 5-6 (Mohs)
Color: Dark grey to black
Luster: Dull to sub-metallic

Identified March 2, 2026More igneous →

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Physical properties

Hardness: 5-6 (Mohs); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: Non-existent; Specific Gravity: 2.8–3.0

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 appears water-worn, suggesting a secondary transport process in a river or coastal environment.

Uses & applications

Commonly used as aggregate in construction (roads/concrete), for industrial stone wool insulation, and occasionally in decorative landscaping or as grounding stones in 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 prevalent on Mars and the Moon.

Field identification & locations

Identify in the field by its heavy weight relative to size and dark, uniform color. It lack visible grains to the naked eye. Often found in volcanic regions or as smooth pebbles in riverbeds.

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

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Magnetite

Magnetite (Fe3O4)

mineral