igneous

Basalt with Quartz/Epidote Vein

Mafic volcanic rock (formula primarily (Mg, Fe)2SiO4 and CaAl2Si2O8)

Hardness: 6-7 Mohs; Color: Black to dark charcoal with a tan/greenish vein; Luster: Dull to sub-vitreous; Structure: Fine-grained (aphanitic) matrix; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8–3.0.

Hardness: 6-7 Mohs
Color: Black to dark charcoal with a tan/greenish vein
Luster: Dull to sub-vitreous

Identified March 28, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. The visible vein was formed later when mineral-rich hydrothermal fluids filled cracks in the cooling basalt. Common in volcanic islands and oceanic crust.

Uses & applications

Basalt is widely used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. This specific tumbled specimen is primarily of interest to hobbyist beachcombers or as decorative 'river rock'.

Geological facts

Basalt is the most common rock type in the Earth's crust and also covers large areas of the Moon (the dark 'Maria' patches) and Mars. This specimen has been naturally tumbled and polished by wave action on a beach.

Field identification & locations

Identify in the field by its heavy weight (density), dark color, and lack of visible large crystals. The lighter band 'vein' is a distinct identifying feature of secondary mineralization. Commonly found on volcanic coastlines in the Pacific Northwest, Hawaii, and Iceland.

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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)

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Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

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Blue Apatite

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

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Magnetite

Magnetite (Fe3O4)

mineral