Igneous

Basalt (River Pebble)

Basalt (Aphanitic Mafic Volcanic Rock)

Hardness: 6-7 on Mohs scale; Color: Dark grey to black; Luster: Dull/Matte (earthy); Crystal structure: Fine-grained/Aphanitic (not visible to naked eye); Cleavage: None (conchoidal fracture); Specific Gravity: 2.8 - 3.0

Hardness: 6-7 on Mohs scale
Color: Dark grey to black
Luster: Dull/Matte (earthy)

Identified March 29, 2026More igneous →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. This specific specimen has been rounded and smoothed by water erosion in a fluvial (river) or coastal environment.

Uses & applications

Commonly used in construction as aggregate (crushed stone), for asphalt pavement, railroad ballast, and in the manufacturing of floor tiles. In landscaping, these smooth pebbles are used for decorative drainage and garden paths.

Geological facts

Basalt is the most common rock type in the Earth's crust, making up most of the ocean floor. It is also extremely common on the Moon (the dark 'Maria' or seas) and Mars.

Field identification & locations

Identify in the field by its high density (heavy for its size), dark uniform color, and lack of visible crystals. It is frequently found as rounded cobbles in riverbeds and on beaches near volcanic regions.

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