igneous

Basalt River Pebble

Basalt (Aphanitic Mafic Volcanic Rock)

Hardness: 6-7 Mohs; Color: Dark grey to charcoal (often appears black when wet); Luster: Dull to earthy; Crystal Structure: Microcrystalline/Aphanitic (crystals too small to see with the naked eye); Cleavage: None; Specific Gravity: 2.8-3.0.

Hardness: 6-7 Mohs
Color: Dark grey to charcoal (often appears black when wet)
Luster: Dull to earthy

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. The rounded shape indicates long-term transport and mechanical weathering in a high-energy fluvial (river) or coastal environment.

Uses & applications

Used extensively in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Smooth river pebbles are highly popular in landscaping, aquarium decor, and zen gardens.

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 on the Moon and Mars, where dark 'seas' or maria are actually vast plains of basaltic lava.

Field identification & locations

Identify by its heavy weight (density), dark uniform color, and lack of visible grains. Common in volcanic regions, riverbeds, and glacial till. Collectors look for unique shapes or vesicles (gas bubbles) filled with secondary minerals like quartz or zeolites.

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