igneous

Amygdaloidal Basalt

Amygdaloidal Basalt (Mafic Extrusive)

Hardness: 5-6 Mohs. Color: Reddish-brown to dark gray groundmass with visible gas cavities (vesicles) filled with secondary minerals like quartz, agate, or chlorite. Luster: Dull/earthy for groundmass, vitreous for fillings. Crystal structure: Aphanitic (fine-grained) groundmass.

Hardness: 5-6 Mohs
Luster: Dull/earthy for groundmass, vitreous for fillings

Identified March 24, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the cooling of iron-rich lava flows. During the cooling process, gas bubbles are trapped in the lava. Later, mineral-bearing groundwater flows through the rock, depositing minerals into these bubbles, turning them into 'amygdules.' Often associated with the Keweenawan Rift (approx. 1.1 billion years old).

Uses & applications

Used primarily as road ballast, crushed stone for construction, and as a source for semi-precious amygdule minerals like Lake Superior Agates for jewelry and lapidary art.

Geological facts

The word 'amygdule' comes from the Greek word for 'almond,' describing the frequent shape of the mineral fillings. Some amygdaloidal basalts in Northern Michigan are famous for containing native copper within the cavities.

Field identification & locations

Identify in the field by looking for a dark, heavy volcanic rock with 'pockmarks' or colorful mineral 'spots' that don't match the surrounding matrix. Common in the Lake Superior region and other ancient volcanic zones. Collectors should look for patterns where the amygdules are distinct and colorful.

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