igneous

Basalt

Basalt (Mafic Extrusive Igneous Rock)

Hardness: 5-6 Mohs; Color: Dark grey to black, greenish when weathered; Luster: Dull to sub-metallic; Structure: Fine-grained aphanitic or porphyritic texture, often featuring columnar jointing or vesicular pores; Cleavage: None/irregular fracture; Specific Gravity: 2.8-3.0.

Hardness: 5-6 Mohs
Color: Dark grey to black, greenish when weathered
Luster: Dull to sub-metallic

Identified March 26, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of low-viscosity magnesium-rich and iron-rich semi-molten lava at or very near the Earth's surface. Basalt is the most common rock in Earth's crust, found in oceanic plates and large volcanic provinces (Traps). Ages range from modern flows to over 3.5 billion years.

Uses & applications

Used extensively in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Also used as dimension stone for pavement and monuments. High-quality basalt is used to manufacture 'stone wool' insulation.

Geological facts

Basalt is not just an Earth rock; it covers much of the surface of the Moon (Maria) and Mars. Some of the most famous formations include the Giant's Causeway in Northern Ireland and the Devils Postpile in California.

Field identification & locations

Identify in the field by its dark color, heavy weight (density), and fine-grained texture where individual crystals are not visible to the naked eye. Look for hexagonal cooling columns or small holes (vesicles) caused by gas bubbles. Commonly found in volcanic regions, oceanic islands (Hawaii, Iceland), 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