igneous

Amygdaloidal Basalt

Vesicular Basalt with secondary mineral infilling (commonly Zeolite, Calcite, or Quartz)

Hardness: 6-7 (groundmass), secondary minerals vary; Color: Dark grey to black groundmass with white or light-colored spots (amygdules); Luster: Dull/earthy matrix with vitreous/pearly inclusions; Texture: Aphanitic with amygdaloidal structure.

Hardness: 6-7 (groundmass), secondary minerals vary
Luster: Dull/earthy matrix with vitreous/pearly inclusions

Identified January 1, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava. Gas bubbles (vesicles) trapped in the cooling lava were later filled with secondary minerals precipitated from hydrothermal fluids or groundwater over millions of years.

Uses & applications

Used primarily as crushed stone for construction, road base, and railroad ballast. Decorative specimens are popular among amateur geologists and rock collectors due to the contrasting 'polka-dot' appearance.

Geological facts

The term 'amygdaloidal' comes from the Greek word 'amygdalo', meaning almond, referring to the almond-like shape of the mineral-filled vesicles. This rock is a common host for native copper in regions like the Keweenaw Peninsula.

Field identification & locations

Identify in the field by looking for a heavy, dark, fine-grained volcanic rock containing distinct, rounded white or colorful mineral 'spots' that don't rub off. Commonly found in volcanic fields, oceanic crust outcrops, and areas like the Lake Superior region or basaltic trap rocks.

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