igneous

Basalt

Mafic volcanic rock (various silicate minerals including Pyroxene and Plagioclase)

Hardness: 6 (Mohs scale); Color: Dark grey to black, often weathering to green or brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None (conchoidal fracture); Specific gravity: 2.8 - 3.0

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

Identified March 21, 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 surface of Earth or a planetary body. It is the most common volcanic rock on Earth, making up most of the ocean floor.

Uses & applications

Used primarily in construction for road base, concrete aggregate, railroad ballast, and dimension stone for paving. Some varieties are used in 'hot stone' therapy or as mineral wool insulation.

Geological facts

Basalt is the most common rock type in the Earth's crust and also makes up the lunar maria on the Moon. When it cools slowly over large areas, it can form unique hexagonal columns like those at the Giant's Causeway.

Field identification & locations

Identify by its heavy weight (high density), dark color, and very fine-grained texture where individual crystals are not visible to the naked eye. Commonly found in volcanic regions, oceanic islands, and mid-ocean ridges. Collectors look for vesicles (gas bubbles) filled with secondary minerals like zeolites or olivine crystals.

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