igneous

Basalt

Basalt (Mafic Extrusive Igneous Rock)

Hardness: 6-7 on Mohs scale; Color: typically dark grey to black, weathering to brown or green; Luster: dull to sub-metallic; Crystal Structure: aphanitic (fine-grained), often containing phenocrysts of plagioclase fieldspar or olivine; Cleavage: poor/none; Specific Gravity: 2.8-3.0.

Hardness: 6-7 on Mohs scale
Color: typically dark grey to black, weathering to brown or green
Luster: dull to sub-metallic

Identified March 29, 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 near the surface of a terrestrial planet or moon. Basalt is found across all geological ages, from Hadean oceanic crust to modern active volcanoes like those in Hawaii or Iceland.

Uses & applications

Extensively used in construction as crushed stone for road base, railroad ballast, and concrete aggregate. High-quality dense basalt is used for 'cast basalt' wear-resistant liners and as a primary material for stone wool insulation.

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 the most common rock on the Moon and Mars. When cooling slowly under specific conditions, it can form striking vertical columns known as columnar jointing.

Field identification & locations

Identify in the field by its heavy weight (density), dark color, and very fine-grained texture. It often shows 'vesicles' (small gas bubbles) or a weathered brown exterior. Commonly found in volcanic fields, oceanic islands, and mid-ocean ridges.

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