igneous

Amygdaloidal Basalt with Quartz/Calcite Inclusions

Amygdaloidal Basalt (SiO2 / CaCO3 inclusions)

Hardness: 5-6 (matrix); Color: Dark brown to reddish-tan weathered exterior (limonite staining); Luster: Dull/Earthy matrix with Vitreous inclusions; Crystal structure: Afanitic matrix with hexagonal (quartz) or rhombohedral (calcite) amygdules.

Hardness: 5-6 (matrix)
Luster: Dull/Earthy matrix with Vitreous inclusions

Identified March 29, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of basaltic lava. Gas bubbles (vesicles) trapped in the cooling lava were later filled with secondary minerals precipitated from hydrothermal fluids passing through the rock.

Uses & applications

Primary use is in construction as road aggregate or railroad ballast; amygdaloidal specimens are popular among amateur collectors and for educational purposes.

Geological facts

The term 'amygdule' comes from the Latin word for 'almond,' describing the shape of the mineral fillings. These rocks are often associated with the Lake Superior region's famous copper deposits.

Field identification & locations

Identify by looking for 'pockmarks' or holes (vesicles) that are filled with lighter-colored minerals against a dark, dense rock background. Commonly found in volcanic terrains like the Pacific Northwest or the Deccan Traps.

Other igneous specimens

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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