Igneous

Basalt

Basalt (dominated by plagioclase feldspar and pyroxene minerals)

Hardness: 6 (Mohs scale); Color: Dark grey to black, weathering to olive-green or brown; Luster: Dull/earthy to sub-metallic; Crystal Structure: Aphanitic (fine-grained) or porphyritic; Specific Gravity: 2.8–3.0

Hardness: 6 (Mohs scale)
Color: Dark grey to black, weathering to olive-green or brown
Luster: Dull/earthy to sub-metallic

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. Basalt is the most common rock type in the Earth's crust (mostly oceanic crust) and can range from modern to billions of years old.

Uses & applications

Used extensively in construction as crushed stone for road base, railroad ballast, and concrete aggregate. It is also used for paving stones, sculptures, and occasionally as 'lava rock' for landscaping or grill beds.

Geological facts

Basalt also exists on other planetary bodies; the lunar maria (dark plains on the moon) and Olympus Mons on Mars are primarily composed of basalt. It often forms striking geometric patterns known as columnar jointing when it cools.

Field identification & locations

Identify in the field by its high density (feels heavy for its size), dark color, and fine-grained texture with no visible large crystals. It often has small 'vesicles' (holes from air bubbles). Common in volcanic regions like Hawaii, the Pacific Northwest, 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