Igneous

Basalt

Mafic volcanic rock, primarily composed of Plagioclase, Pyroxene, and Olivine

Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-metallic (glassy when quenched); Crystal Structure: Fine-grained (aphanitic) or porphyritic; Specific Gravity: 2.8–3.0

Hardness: 6 (Mohs scale)
Color: Dark grey to black
Luster: Dull to sub-metallic (glassy when quenched)

Identified March 25, 2026More igneous →

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Physical properties

Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-metallic (glassy when quenched); Crystal Structure: Fine-grained (aphanitic) or porphyritic; Specific Gravity: 2.8–3.0

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the surface of a terrestrial planet or moon. Most basalt flows are from the Cenozoic era, though it has formed throughout Earth's history.

Uses & applications

Used extensively in construction as crushed stone for road base, concrete aggregate, and railroad ballast. It is also used in high-quality floor tiles and as 'hot stones' in massage therapy.

Geological facts

Basalt is the most common rock type in the Earth's crust and makes up the majority of the ocean floor. It is also found on the Moon and Mars, forming the dark 'maria' plains visible from Earth.

Field identification & locations

Identify in the field by its dark color, heavy weight (density), and lack of visible crystals to the naked eye. Commonly found in volcanic regions like Hawaii, Iceland, or the Deccan Traps. Collectors look for vesicles (gas bubbles) that may contain secondary minerals like zeolites or amethysts.

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