igneous

Basalt

Basalt (Mafic volcanic rock)

Hardness: 6-7 Mohs; Color: Dark grey to black; Luster: Dull to earthy; Texture: Aphanitic (fine-grained) with small visible phenocrysts; Structure: Dense and massive; Specific Gravity: ~2.8-3.0

Hardness: 6-7 Mohs
Color: Dark grey to black
Luster: Dull to earthy

Identified February 8, 2026More igneous →

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Physical properties

Hardness: 6-7 Mohs; Color: Dark grey to black; Luster: Dull to earthy; Texture: Aphanitic (fine-grained) with small visible phenocrysts; Structure: Dense and massive; 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 appears water-worn, suggesting it was found in a riverbed or coastal environment. It originates from volcanic activity across many geological eras.

Uses & applications

Extensively used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. In high-density forms, it is used for floor tiles and cobblestones. Some varieties are used in 'stone washing' textiles or as heat-retaining sauna stones.

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 prevalent on other planetary bodies including the Moon, Mars, and Venus. The Giant's Causeway in Ireland is a famous example of columnar basalt jointing.

Field identification & locations

Identify by its heavy weight (high density), dark color, and fine-grained texture. To distinguish from slate, check for a lack of layering. It is commonly found in volcanic fields, oceanic islands (like Hawaii), and mid-ocean ridges. Collectors look for unweathered surfaces to see the true dark color.

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