igneous

Basalt (Weathered Pebble)

Extrusive mafic igneous rock; minerals primarily Plagioclase feldspar and Pyroxene

Hardness: 6 (Mohs scale); Color: Dark grey to greenish-grey (weathered); Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8 - 3.0

Hardness: 6 (Mohs scale)
Color: Dark grey to greenish-grey (weathered)
Luster: Dull/Earthy

Identified March 24, 2026More igneous →

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Physical properties

Hardness: 6 (Mohs scale); Color: Dark grey to greenish-grey (weathered); Luster: Dull/Earthy; 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. This specific specimen appears to be a water-worn or weathered pebble likely from a riverbed or coastal environment.

Uses & applications

Primary use is in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Fine specimens are sometimes used in lapidary work as 'river stones' for landscaping or grounding tools.

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 found extensively on the Moon and Mars.

Field identification & locations

Identify by its dark color, heavy weight for its size (density), and fine-grained texture. It is commonly found in volcanic regions, oceanic islands (like Hawaii), and riverbeds downstream from volcanic plateaus.

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