igneous

Amygdaloidal Rhyolite (often called Lake Superior Agate relative)

Rhyolite with Calcite/Quartz amygdules (SiO2 and secondary minerals)

Hardness: 6-7 (Mohs scale); Color: Grayish-purple matrix with white or pale green inclusions; Luster: Dull/waxy matrix, vitreous inclusions; Structure: Aphanitic with vesicles filled by secondary minerals (amygdules).

Hardness: 6-7 (Mohs scale)
Color: Grayish-purple matrix with white or pale green inclusions
Luster: Dull/waxy matrix, vitreous inclusions

Identified March 29, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of silica-rich lava. The 'amygdules' (white spots) formed when gas bubbles (vesicles) were later filled by minerals like quartz or calcite as groundwater moved through the rock. Likely from the Midcontinent Rift system (approx. 1.1 billion years ago).

Uses & applications

Used primarily for lapidary work (tumbling and polishing), jewelry making as semi-precious stones, and as geological teaching specimens.

Geological facts

Many of these specimens are found on the shores of Lake Superior, having been transported hundreds of miles by glacial movement during the last Ice Age. While they are often found near agates, they are technically volcanic rocks containing mineral deposits.

Field identification & locations

Identify in the field by the distinct 'spotty' appearance of white mineral pockets against a darker, fine-grained purple/gray volcanic background. Common on Great Lakes beaches, particularly in Minnesota and Michigan.

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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)

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Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

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Blue Apatite

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

mineral

Magnetite

Magnetite (Fe3O4)

mineral