igneous

Amygdaloidal Basalt

Amygdaloidal Basalt (Mafic Volcanic Rock)

Hardness: 5-6 (base) / 6-7 (mineral infill); Color: Dark grey or greenish-grey groundmass with white/tan almond-shaped inclusions; Luster: Dull/earthy matrix with vitreous inclusions; Crystal structure: Aphanitic (fine-grained) with secondary mineral infilling.

Hardness: 5-6 (base) / 6-7 (mineral infill)
Luster: Dull/earthy matrix with vitreous inclusions

Identified May 21, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the cooling of gas-rich basaltic lava. As the lava cools, gas bubbles leave behind vesicles. Over geological time, groundwater carries dissolved minerals like quartz, calcite, or zeolites through the rock, precipitating them inside these holes (amygdules). Common in Cenozoic and Mesozoic volcanic flows.

Uses & applications

Primarily used as a decorative specimen for collectors or as landscaping stone. Historically, the groundmass of basalt is used in road construction and as an industrial abrasive.

Geological facts

The term 'amygdaloidal' comes from the Latin 'amygdala', meaning almond, referring to the characteristic shape of the mineral-filled vesicles. This specific specimen appears to be water-worn, likely found in a riverbed or on a beach.

Field identification & locations

Identify in the field by looking for a dark, heavy volcanic rock featuring distinct light-colored 'dots' or 'almonds' of mineral. Found in volcanic regions like the Lake Superior basin, the Pacific Northwest, and Iceland.

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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

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

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Blue Apatite

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

mineral

Magnetite

Magnetite (Fe3O4)

mineral