igneous

Basalt

Mafic volcanic rock (primarily plagioclase feldspar and pyroxene)

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

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

Identified March 23, 2026More igneous →

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Physical properties

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

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava (mafic lava) at or very near the Earth's surface. Found in oceanic crust, mid-ocean ridges, and continental flood basalts across all geological ages.

Uses & applications

Used extensively in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Also used in high-temp insulation (rock wool) and occasionally as 'hot stones' in spa treatments.

Geological facts

Basalt is the most common rock type in the Earth's crust, covering most of the ocean floor. It is also common on other planetary bodies like the Moon (lunar maria), Mars, and Venus.

Field identification & locations

Identify in the field by its high density, dark color, and very fine-grained texture that lacks visible crystals to the naked eye. Commonly found in volcanic regions, cliffs, and riverbeds.

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

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