igneous

Amygdaloidal Basalt

Basalt with Secondary Mineral Infillings

Hardness: 5-7 on Mohs scale; Color: Dark grey to reddish-brown matrix with white or light-colored circular spots; Luster: Dull (matrix) to vitreous (fillings); Crystal structure: Aphanitic (fine-grained) with larger mineral nodes.

Hardness: 5-7 on Mohs scale
Luster: Dull (matrix) to vitreous (fillings)

Identified April 10, 2026More igneous →

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Physical properties

Formation & geological history

Formed from volcanic lava flows where gas bubbles (vesicles) were trapped. Over geological time, groundwater carrying dissolved minerals like quartz, calcite, or zeolites percolated through the rock and filled the cavities.

Uses & applications

Commonly used in landscaping, as road aggregate, or as decorative specimens for lapidary work and collectors.

Geological facts

The word 'Amygdaloid' comes from the Greek word 'amygdale' meaning almond, referring to the almond-like shape of the mineral fillings in the rock.

Field identification & locations

Identify in the field by looking for a fine-grained, dark volcanic rock peppered with distinct 'polka dots' of lighter mineral. Commonly found in the Lake Superior region of North America and volcanic fields worldwide.

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