igneous

Basalt (River Pebble)

Basalt

Hardness: 6 (Mohs scale); Color: Dark grey to black, weathering to brown; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained); Specific Gravity: 2.8–3.0

Hardness: 6 (Mohs scale)
Color: Dark grey to black, weathering to brown
Luster: Dull to sub-metallic

Identified March 16, 2026More igneous →

Identify your own rocks.

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

App Store Google Play

Physical properties

Hardness: 6 (Mohs scale); Color: Dark grey to black, weathering to brown; Luster: Dull to sub-metallic; 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 rounded by fluvial (river) erosion over a period of decades to centuries.

Uses & applications

Used extensively in construction as aggregate, for road base, and in concrete. Polished pebbles are used in landscaping, 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 on the Moon, Mars, and Venus.

Field identification & locations

Identify in the field by its dark color, heavy weight (relative to size), and fine-grained texture. It often shows a smooth, water-worn surface if found in riverbeds or coastal areas. Very common in volcanic regions and glacial deposits.

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