Igneous

River Basalt (Cobble)

Mafic Igneous Rock (Fine-grained Basalt)

Hardness: 5-6 (Mohs); Color: Dark gray, charcoal, or greenish-black; Luster: Dull to earthy (unless polished or wet); Crystal Structure: Aphanitic (fine-grained); Specific Gravity: 2.8–3.0.

Hardness: 5-6 (Mohs)
Color: Dark gray, charcoal, or greenish-black
Luster: Dull to earthy (unless polished or wet)

Identified March 28, 2026More igneous →

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Physical properties

Hardness: 5-6 (Mohs); Color: Dark gray, charcoal, or greenish-black; Luster: Dull to earthy (unless polished or 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. This specific specimen has been tumbled and rounded by hydraulic action in a river or stream over hundreds to thousands of years.

Uses & applications

Commonly used as crushed stone in construction, road base, and railroad ballast. Smooth river stones are used in landscaping, decorative aquariums, and hot stone massage therapy.

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 in the field by its high density/weight, dark color, and lack of visible crystals to the naked eye. Look for smooth, rounded edges and a waxy appearance when wet. Very common in volcanic regions and riverbeds.

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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

mineral

Blue Apatite

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

mineral

Magnetite

Magnetite (Fe3O4)

mineral