Igneous

Amygdaloidal Basalt

Amygdaloidal Basalt (Mafic silicate with secondary mineral inclusions)

Hardness: 5-6 (base rock), 7 (quartz/chalcedony inclusions); Color: Dull brownish-gray to dark green matrix with circular voids or mineral fillings; Luster: Earthy to sub-metallic; Crystal structure: Aphanitic (fine-grained) matrix.

Hardness: 5-6 (base rock), 7 (quartz/chalcedony inclusions)
Luster: Earthy to sub-metallic

Identified March 29, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of mafic lava flows. Gas bubbles (vesicles) are trapped as the lava hardens; later, groundwater deposits secondary minerals like quartz, calcite, or zeolites into these holes, forming the almond-shaped 'amygdules.'

Uses & applications

Commonly used as crushed stone for road construction and concrete aggregate. Attractive specimens are used in decorative stone landscaping or lapidary work (tumbling and polishing).

Geological facts

The word 'amygdaloidal' comes from the Greek word 'amygdale,' meaning almond, referring to the shape of the mineral fillings. These rocks are often the source for Lake Superior Agates.

Field identification & locations

Identify by looking for a dark, dense, fine-grained igneous rock containing distinct, rounded pits or solid mineral 'spots' of a different color. Commonly found in volcanic regions such as the Keweenawan Peninsula of Michigan or the basaltic flows of the Pacific Northwest.

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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

Blue Apatite

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

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral