igneous

Basalt Pebble

Basalt (Mafic Extrusive Igneous Rock)

Hardness: 6-7 on Mohs scale; Color: Dark grey to black, often appearing darker when wet; Luster: Dull to sub-vitreous; Crystal Structure: Aphanitic (fine-grained); Cleavage: None; Specific Gravity: 2.8-3.0.

Hardness: 6-7 on Mohs scale
Color: Dark grey to black, often appearing darker when wet
Luster: Dull to sub-vitreous

Identified March 29, 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. This specific specimen has been basalt water-worn into a smooth pebble by fluvial or coastal erosion over hundreds to thousands of years.

Uses & applications

Predominantly used as crushed stone for construction, road base, and railroad ballast. Smooth river-worn specimens are popular for landscaping, decorative gardens, and hot stone massage therapy.

Geological facts

Basalt is the most common rock type on Earth's crust and makes up most of the ocean floor. It is also found extensively on the Moon (the dark 'Maria') and Mars.

Field identification & locations

Identify in the field by its dark color, heavy weight (dense), and lack of visible crystals to the naked eye. Commonly found on volcanic beaches, riverbeds near volcanic ranges, and oceanic islands like Iceland or Hawaii.

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