igneous

Basalt

Basalt (Mafic volcanic rock)

Hardness: 5-6 Mohs. Color: Dark grey to black, weathering to brown or green. Luster: Dull to sub-metallic. Structure: Fine-grained (aphanitic), often contains small white plagioclase crystals. Specific gravity: 2.8-3.0.

Hardness: 5-6 Mohs
Color: Dark grey to black, weathering to brown or green
Luster: Dull to sub-metallic

Identified May 25, 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. It is the most common volcanic rock on Earth, comprising most of the ocean floor and large volcanic provinces.

Uses & applications

Extensively used in construction as crushed stone for road base, railroad ballast, and concrete aggregate. High-quality specimens are also used as 'hot stones' in massage therapy and for stone carvings.

Geological facts

Basalt is not unique to Earth; it is the most common rock type on the Moon (the dark lunar maria) and covers a significant portion of Mars and Venus.

Field identification & locations

Identified by its heavy weight (high density), dark color, and fine-grained texture. To identify in the field, look for hexagonal columnar jointing or vesicular (pitted) surfaces. Commonly found in volcanic regions like 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