igneous

Basalt

Mafic Volcanic Rock (Primarily Plagioclase Feldspar and Pyroxene)

Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Variable, often dull but can be waxy or glassy in fine-grained specimens; Crystal Structure: Aphanitic (fine-grained, crystals too small to see with the naked eye); Specific Gravity: 2.8 - 3.0

Hardness: 6 (Mohs scale)
Color: Dark grey to black

Identified March 16, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of low-viscosity lava rich in magnesium and iron at or very near the Earth's surface. Most basalt is Cenozoic or Mesozoic in age, though it has formed throughout Earth's history.

Uses & applications

Extensively used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. It is also used as dimension stone and occasionally in sculpture or as a base for jewelry.

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 the Moon, Mars, and Venus, where it forms vast volcanic plains.

Field identification & locations

Identify in the field by its dark weight and density compared to other rocks; it often lacks visible crystals and may show small bubbles (vesicles) or a smooth, worn surface in riverbeds. Common in volcanic regions such as the Pacific Northwest, Hawaii, and Iceland.

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