igneous

Basalt with Olivine Phenocrysts

Porphyritic Basalt (with (Mg, Fe)2SiO4)

Hardness: 6-7 (Mohs scale), Color: Dark grey to black matrix with olive-green or brownish inclusions, Luster: Vitreous to dull, Crystal structure: Orthorhombic (olivine), Cleavage: Indistinct, Specific Gravity: 2.8-3.4

Identified April 7, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava (basaltic) either at or very near the Earth's surface during volcanic eruptions. Common in oceanic crust and volcanic islands.

Uses & applications

Used primarily as crushed stone for road base, concrete aggregate, and railroad ballast. Gem-quality olivine (peridot) is used in jewelry.

Geological facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. When olivine crystals are large and clear enough, they are the source of the August birthstone, Peridot.

Field identification & locations

Identify in the field by its dark, dense sugary-textured matrix and the presence of small, glassy green 'sand-like' grains. It is commonly found in volcanic regions such as Hawaii, Iceland, or the Columbia River Plateau.

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