igneous

Basalt with Limonite stains

Extrusive mafic igneous rock; Limonite (FeO(OH)·nH2O)

Hardness: 6 (Mohs scale), Color: Dark grey to charcoal with yellow-brown oxidation, Luster: Dull/Earthy, Crystal Structure: Aphanitic (fine-grained), Cleavage: None/Conchoidal fracture

Identified March 18, 2026More igneous →

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Physical properties

Hardness: 6 (Mohs scale), Color: Dark grey to charcoal with yellow-brown oxidation, Luster: Dull/Earthy, Crystal Structure: Aphanitic (fine-grained), Cleavage: None/Conchoidal fracture

Formation & geological history

Formed from the rapid cooling of magnesium and iron-rich lava at the Earth's surface. The orange-brown staining is caused by the chemical weathering and hydration of iron-rich silicate minerals (like olivine or pyroxene) into limonite.

Uses & applications

Primary use in construction as aggregate, road base, and concrete. In small specimens, it is used for geological teaching and children's starter collections.

Geological facts

Basalt is the most common rock type on the Earth's surface and actually makes up most of the ocean floor. It is also found in abundance on the Moon and Mars.

Field identification & locations

Identify by its dark, heavy, fine-grained mass and the presence of rusty oxidation. Common in volcanic regions worldwide (e.g., Pacific Northwest USA, Iceland, Hawaii). Collectors look for vesicles or 'holes' containing secondary crystals like zeolites.

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

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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

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Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Magnetite

Magnetite (Fe3O4)

mineral