igneous

Amygdaloidal Basalt

Amygdaloidal Basalt (Mafic extrusive volcanic rock with secondary mineral infilling)

Hardness: 5-6 (basalt matrix); Color: Dark grey, reddish-brown, or black with white, pink, or green vesicles; Luster: Dull to earthy; Crystal structure: Aphanitic (fine-grained) matrix; Cleavage: None/Irregular fracture.

Hardness: 5-6 (basalt matrix)
Luster: Dull to earthy

Identified March 29, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of basaltic lava. During cooling, gas bubbles (vesicles) are trapped in the rock. Later, mineral-rich groundwater flows through these voids, depositing minerals like quartz, calcite, zeolites, or epidote to create the 'amygdules' (the light-colored spots).

Uses & applications

Used primarily as crushed stone for road construction and concrete aggregate. Attractive specimens are popular for amateur geology collections and educational purposes.

Geological facts

The word 'amygdaloid' comes from 'amygdala,' the Latin word for almond, because the mineral-filled pits often resemble almonds. This rock type is common in the Lake Superior region of North America and contains the world's largest deposits of native copper.

Field identification & locations

Identify in the field by looking for a heavy, dark, fine-grained rock with round or oval 'polka dots' of a different mineral. Can be found in volcanic regions like the Pacific Northwest, Lake Superior, or Iceland.

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

Blue Apatite

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

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral