igneous

Basalt Pebble

Extrusive mafic igneous rock; primarily (Ca, Na)(Mg, Fe, Al)Si2O6 and (Ca, Na)AlSi3O8

Hardness: 5-6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific gravity: 2.8–3.0

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

Identified March 20, 2026More igneous →

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Physical properties

Hardness: 5-6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific gravity: 2.8–3.0

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at the earth's surface. This specific specimen has been rounded and smoothed by water erosion (fluvial or glacial action) during the Cenozoic era.

Uses & applications

Used primarily in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Smooth basalt pebbles are popular in landscaping and decorative masonry.

Geological facts

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

Field identification & locations

Identify in the field by its heavy weight (density), dark uniform color, and fine grain. Commonly found in volcanic regions, riverbeds, and coastal areas downstream from volcanic flows.

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