Igneous

Basalt Pebble

Basalt (Mafic volcanic rock)

Hardness: 5-6 (Mohs scale); Color: Dark grey to black, sometimes with greenish or reddish weathering; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None, often demonstrates conchoidal fracture under the weathered surface; Specific Gravity: 2.8–3.0.

Hardness: 5-6 (Mohs scale)
Luster: Dull to sub-metallic

Identified March 27, 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. This specimen has been rounded by fluvial (river) or coastal (beach) erosion. Basalts can range from recent volcanic activity to billions of years old.

Uses & applications

Commonly used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. In polished or pebble form, it is popular for landscaping, aquarium decoration, 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. The Giant's Causeway in Northern Ireland is a famous example of columnar basalt.

Field identification & locations

Identify in the field by its high density (feels heavy for its size), dark color, and lack of visible large crystals. It is ubiquitous along coastlines with volcanic history or in riverbeds draining volcanic regions. Collectors look for vesicles (gas bubbles) or phenocrysts (larger embedded crystals) like green olivine.

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