igneous

Basalt with Weathered Rind

Basalt (Aphanitic Mafic Igneous Rock)

Hardness: 6 (Mohs scale); Color: Dark grey to charcoal black (interior), tan/brown (weathered exterior); Luster: Dull to sub-metallic; Crystal Structure: Microcrystalline/Aphanitic; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8 - 3.0

Hardness: 6 (Mohs scale)
Luster: Dull to sub-metallic

Identified March 22, 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 very near the Earth's surface. Basalt is the most common volcanic rock on Earth and forms much of the ocean floor and large plateau regions like the Deccan Traps.

Uses & applications

Extensively used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. In finer forms, it is used for floor tiles, monuments, and 'hot stone' massage therapy due to its high thermal mass.

Geological facts

Basalt is not just Earth-bound; it is the most common rock on the Moon's surface, forming the lunar maria (the dark plains seen from Earth). It also covers much of the surface of Mars and Venus.

Field identification & locations

Identify in the field by its heavy weight (density), dark interior, and fine-grained texture that lacks visible crystals to the naked eye. Often found in volcanic regions or as 'river rocks' transported by water. Use a magnet to test for high iron content; some basalt specimens are slightly magnetic.

Other igneous specimens

See all

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