igneous

Basalt

Basalt (Mafic volcanic rock)

Hardness: 6.0 on Mohs scale; Color: Dark grey to black, often weathering to brown or olive green; Luster: Dull (earthy) to sub-metallic; Crystal structure: Aphanitic (fine-grained, crystals too small to see with the naked eye); Cleavage: None; Specific gravity: 2.8 - 3.0 (dense).

Hardness: 6
Color: Dark grey to black, often weathering to brown or olive green
Luster: Dull (earthy) to sub-metallic

Identified March 2, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of low-viscosity lava rich in magnesium and iron at or very near the Earth's surface. Basalt is the primary rock type underlying the Earth's ocean basins and is commonly found in volcanic regions. Much of the oceanic crust is less than 200 million years old, though continental basalt flows can be much older.

Uses & applications

Extensively used in construction as crushed stone for road bases, concrete aggregate, and railroad ballast. It is also used as a dimension stone for floor tiles and in the production of basalt fiber used for reinforcing plastics.

Geological facts

Basalt is the most common rock type in the Earth's crust and also makes up most of the surface of the Moon (the dark 'seas' or maria) and Mars. Some basalt displays 'columnar jointing,' creating spectacular hexagonal pillars like those at the Giant's Causeway.

Field identification & locations

Identify in the field by its dark color, heavy weight (high density), and fine-grained texture that lacks visible crystals. It common in the Pacific Northwest of the US, Hawaii, Iceland, and India's Deccan Traps. Collectors often look for basalt containing vesicles (gas bubbles) filled with secondary minerals like zeolites or amethyst.

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