igneous

Basalt with Iron Oxidation

Mafic Igneous Rock (Basalt)

Hardness: 6 (Mohs scale); Color: Dark grey to black interior with reddish-brown exterior weathering; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8 - 3.0

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

Identified March 17, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at the Earth's surface. The outer reddish hue is caused by chemical weathering (oxidation) of iron-bearing minerals over decades to centuries.

Uses & applications

Commonly used in construction as an aggregate for road base, concrete, and railroad ballast. Larger slabs are used for architectural masonry. Occasional use in stone massage therapy.

Geological facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. Large-scale eruptions of basalt are known as 'Flood Basalts' and can cover thousands of square miles.

Field identification & locations

Identify by its heavy weight (high density), dark fine-grained interior, and characteristic rusty-orange weathering crust. Look for conchoidal to irregular fracture patterns. Common in volcanic regions or glacial till.

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

Blue Apatite

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

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral