igneous

Basalt

Extrusive Mafic Igneous Rock (composed primarily of plagioclase feldspar and pyroxene)

Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull to sub-vitreous; Structure: Aphanitic (fine-grained) to vesicular; Cleavage: None/Conchoidal fracture; Specific gravity: 2.8 - 3.0

Hardness: 6 (Mohs scale)
Color: Dark gray to black
Luster: Dull to sub-vitreous

Identified March 13, 2026More igneous →

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Physical properties

Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull to sub-vitreous; Structure: Aphanitic (fine-grained) to vesicular; Cleavage: None/Conchoidal fracture; Specific gravity: 2.8 - 3.0

Formation & geological history

Formed from the rapid cooling of low-viscosity magnesium-rich and iron-rich lava at or very near the Earth's surface. It can be found in volcanic arcs, oceanic islands, and mid-ocean ridges across all geological ages.

Uses & applications

Extensively used in construction as aggregate (crushed stone), for road base, railroad ballast, and high-quality concrete. It is also used as a raw material for stone wool insulation and occasionally used in ornamental landscaping.

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 found on other planetary bodies like the Moon (lunar maria), Mars, and Venus.

Field identification & locations

Identify in the field by its heavy weight (relative to size), dark color, and fine-grained texture where individual crystals are not visible to the naked eye. Often exhibits 'vesicles' (small holes from gas bubbles) or column-like jointing in large formations.

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