igneous

River Pebble (Gabbro or Basalt)

Mafic Igneous Rock (composed primarily of Pyroxene and Plagioclase Feldspar)

Hardness: 6-7 (Mohs scale), Color: Dark grey to black with salt-and-pepper flecks, Luster: Dull naturally but vitreous/shiny when polished or wet, Crystal structure: Phaneritic (visible grains in Gabbro) to Aphanitic (fine grains in Basalt), Specific gravity: 2.8-3.3

Identified April 4, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the slow cooling of magnesium-rich and iron-rich magma beneath the Earth's surface (Gabbro) or rapid cooling of lava flow (Basalt). These specimens have been smoothed by fluvial (river) or coastal erosion processes.

Uses & applications

Commonly used in landscaping, as decorative stones in aquariums, or as aggregates in construction and road building. Tumbled versions are used in massage therapy (hot stones) and hobbyist stone collecting.

Geological facts

Gabbro is the intrusive equivalent of Basalt and makes up a significant portion of the Earth's oceanic crust. These rounded shapes are 'well-sorted' by water action, meaning nature has already performed the 'tumbling' process over years of travel downstream.

Field identification & locations

Identify in the field by the high density (heavy for its size), dark color, and lack of visible quartz. Found in riverbeds or beaches near volcanic or tectonic regions. Collectors look for unique banding or 'veining' in these pebbles.

Other igneous specimens

See all

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