igneous

Amygdaloidal Basalt

Amygdaloidal Basalt containing Zeolite or Quartz (SiO2/Silicates)

Hardness: 5-7 (Mohs); Color: Dark grey/black matrix with white or light blue circular inclusions; Luster: Dull/earthy matrix with vitreous/pearly vesicles; Structure: Aphanitic with vesicular/amygdaloidal texture.

Hardness: 5-7 (Mohs)
Luster: Dull/earthy matrix with vitreous/pearly vesicles

Identified March 29, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the cooling of mafic lava flows. Gasses escaping the lava create bubbles (vesicles). Over millions of years, mineral-rich groundwater deposits minerals like quartz, calcite, or zeolites into these holes, creating 'amygdules'.

Uses & applications

Common basalt is used in road construction and concrete aggregate. Amygdaloidal specimens are primarily for geological study and mineral collecting. Polished specimens are occasionally used in lapidary work.

Geological facts

The word 'amygdule' is derived from the Greek word 'amygdala', meaning almond, referring to the almond-shaped mineral deposits often found in these volcanic rocks.

Field identification & locations

Identify by looking for a dark, heavy, fine-grained volcanic rock containing distinct, round 'spots' of a different material. Common in volcanic regions like the Lake Superior Basin, Iceland, and 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