igneous

Basalt with Weathered Crust

Aphanitic Mafic Igneous Rock (Basalt)

Hardness: 5-6 (Mohs); Color: Dark grey to black interior, reddish-brown weathered exterior; Luster: Dull to sub-metallic; Crystal structure: Fine-grained (aphanitic); Cleavage: None; Specific gravity: 2.8-3.0

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

Identified May 28, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or moon. The reddish exterior is due to oxidation (rusting) of iron-bearing minerals.

Uses & applications

Commonly used as aggregate in construction, road base, concrete, and railroad ballast. Exceptionally durable for high-traffic paving.

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.

Field identification & locations

Identify by its heavy weight (high density), dark interior fine-grain texture, and often a brown weathered 'rind'. Found globally in volcanic regions like Hawaii, Iceland, and the Pacific Northwest.

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

Dalmatian Jasper

Aplite with Arfvedsonite inclusions

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