igneous

Basalt

Basalt (Igneous Volcanic Rock)

Hardness: 6 (Mohs scale); Color: Dark gray to black (often weathers to brown or rust-green); Luster: Dull/earthy to sub-metallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; Specific gravity: 2.8–3.0.

Hardness: 6 (Mohs scale)
Color: Dark gray to black (often weathers to brown or rust-green)
Luster: Dull/earthy to sub-metallic

Identified March 21, 2026More igneous →

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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. Basaltic volcanism has occurred throughout Earth's history, from the Archean eon to active eruptions today.

Uses & applications

Extensively used in construction as crushed stone for road base, railroad ballast, and concrete aggregate. High-quality dense basalt is used for floor tiles, monuments, and stone veneers. It is also used in the production of stone wool insulation.

Geological facts

Basalt is the most common rock type in the Earth's crust, covering most of the ocean floor. It is most famous for forming 'columnar jointing' (geometrical pillars) like the Giant's Causeway. It also makes up the vast majority of 'maria' (dark plains) on the Moon.

Field identification & locations

Identify in the field by its high density, dark color, and fine-grained texture that lacks visible crystals to the naked eye. It often shows 'vesicles' (small gas bubbles) or a weathered brownish outer rind. Found worldwide in volcanic regions, oceanic islands (like Hawaii), and mid-ocean ridges.

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