igneous

Basalt Cobble

Aphanitic Mafic Igneous Rock (Basalt)

Hardness: 6.0 on Mohs scale; Color: Dark grey to black or brownish-black; Luster: Dull to sub-metallic (shiny when wet); Crystal Structure: Aphanitic (fine-grained); Specific Gravity: 2.8–3.0

Hardness: 6
Color: Dark grey to black or brownish-black
Luster: Dull to sub-metallic (shiny when wet)

Identified February 20, 2026More igneous →

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Physical properties

Hardness: 6.0 on Mohs scale; Color: Dark grey to black or brownish-black; Luster: Dull to sub-metallic (shiny 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 very near the Earth's surface. Extrusive volcanic origin, commonly found in oceanic crust and lava flows.

Uses & applications

Extensively used in construction for aggregate, road base, and railroad ballast. Smooth river-worn specimens are popular for decorative landscaping and hot stone massage therapy.

Geological facts

Basalt is the most common rock type on Earth's surface and also makes up much of the surface of the Moon and Mars. It is the primary rock of the oceanic crust.

Field identification & locations

Identified by its heavy weight (high density), dark matte color, and lack of visible crystals to the naked eye. Often found on beaches or in riverbeds as rounded 'river stones' or cobbles.

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

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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

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Blue Apatite

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

mineral

Magnetite

Magnetite (Fe3O4)

mineral