igneous

Basalt with Iron Oxidation

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

Hardness: 5-6 (Mohs scale); Color: Dark grey to black interior, dull reddish-brown (limonitic) weathered exterior; Luster: Dull to sub-metallic; Crystal Structure: Fine-grained (microcrystalline); Specific Gravity: 2.8–3.0.

Hardness: 5-6 (Mohs scale)
Luster: Dull to sub-metallic

Identified March 28, 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. The visible reddish crust is a result of chemical weathering where iron minerals oxidize in the presence of water and oxygen.

Uses & applications

Used extensively in construction as aggregate (crushed stone), road base, and railroad ballast. Dense varieties are used for high-quality floor tiles and statues.

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 extensively on the Moon and Mars, forming the dark 'maria' plains visible from Earth.

Field identification & locations

Identify in the field by its high density relative to other dark stones and its lack of visible large crystals (aphanitic). Look for the characteristic orange-brown 'rind' of rust on the exterior. Common worldwide in volcanic seafloors and continental flood basalt provinces.

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