igneous

Amygdaloidal Basalt

Amygdaloidal Mafic Volcanic Rock

Hardness: 6-7 (Mohs); Color: Dark grey to reddish-brown matrix with light white/green mineral fillings; Luster: Dull to earthy (matrix), Vitreous (fillings); Crystal Structure: Aphanitic (fine-grained) with secondary mineral crystal clusters; Specific Gravity: 2.8–3.0.

Hardness: 6-7 (Mohs)
Luster: Dull to earthy (matrix), Vitreous (fillings)

Identified March 4, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the cooling of gas-rich lava. As the lava cools, gas bubbles (vesicles) are trapped; later, mineral-rich groundwater flows through these holes, depositing minerals like quartz, calcite, or epidote to fill them. Often Proterozoic in age (approx. 1 billion years old in the Great Lakes region).

Uses & applications

Primary use as decorative landscaping stones and beach-combing collectibles. Occasionally used in construction aggregate or polished for lapidary work and unique jewelry cabochons.

Geological facts

The term 'amygdule' comes from the Greek word 'amygdalo', meaning 'almond', referring to the almond shape of the mineral fillings. These rocks are most famous for being the primary host rock of native copper in Michigan's Keweenaw Peninsula.

Field identification & locations

Identified in the field by its 'polka-dot' appearance caused by light-colored mineral circles against a darker, weathered volcanic crust. Commonly found on beaches of Lake Superior or mixed into glacial till throughout the American Midwest.

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