igneous

Amygdaloidal Basalt

Amygdaloidal Basalt (Mafic volcanic rock)

Hardness: 5-6 (Mohs scale); Color: Dark grey to purple-black matrix with white or light-colored circular infillings; Luster: Dull (matrix) to vitreous (fillings); Crystal structure: Aphanitic (fine-grained) with secondary mineral amygdules; Specific gravity: 2.8–3.0

Hardness: 5-6 (Mohs scale)
Luster: Dull (matrix) to vitreous (fillings)

Identified May 10, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of basaltic lava. Gas bubbles (vesicles) trapped in the cooling lava are later filled with secondary minerals like calcite, quartz, or zeolites through hydrothermal precipitation or groundwater percolation.

Uses & applications

Used primarily in geological educational collections, as aggregate in construction when found in large volumes, and sometimes as a decorative landscape stone.

Geological facts

The name comes from the Greek word 'amygdale' (almond), referring to the almond-shaped mineral deposits within the rock. This specific appearance is common in the Lake Superior region and ancient volcanic flows.

Field identification & locations

Identify in the field by looking for a dark, dense volcanic matrix containing small, rounded, distinct white or green 'dots'. Commonly found in areas with historic volcanic activity, such as the Keweenaw Peninsula in Michigan or the Deccan Traps.

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