igneous

Basalt

Mafic Igneous Rock (composed primarily of Pyroxene and Plagioclase Feldspar)

Hardness: 6-7 (Mohs scale); Color: Dark grey to black, often weathering to green or brown; Luster: Dull/Sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Specific Gravity: 2.8-3.0.

Hardness: 6-7 (Mohs scale)
Color: Dark grey to black, often weathering to green or brown
Luster: Dull/Sub-metallic

Identified January 13, 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 near the surface of a terrestrial planet or moon. Basalt comprises most of the ocean floor and is found across all geological ages, notably in Large Igneous Provinces.

Uses & applications

Used extensively in construction as crushed stone for road base, concrete aggregate, and railroad ballast. It is also used in the manufacturing of stone wool (insulation material) and occasionally as a high-durability floor tile.

Geological facts

Basalt is the most common rock on the Earth's surface and also covers large areas of the Moon, Mars, and Venus. The Giant's Causeway in Northern Ireland is a famous example of columnar basalt-jointing.

Field identification & locations

Identify in the field by its dark color, heavy weight (relative to size), and fine-grained texture which lacks visible crystals to the naked eye. Commonly found 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

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Magnetite

Magnetite (Fe3O4)

mineral