igneous

Amygdaloidal Basalt with Zeolite/Quartz Infill

Amygdaloidal Basalt (Host: (Mg,Fe)2SiO4 / CaAl2Si2O8; Infill: SiO2 or Na2Al2Si3O10·2H2O)

Hardness: 5-6 (Basalt) / 7 (Quartz). Color: Dark gray to black matrix with white or translucent circular inclusions. Luster: Dull/earthy matrix with vitreous (glassy) amygdules. Structure: Vesicular texture filled with secondary minerals.

Hardness: 5-6 (Basalt) / 7 (Quartz)
Luster: Dull/earthy matrix with vitreous (glassy) amygdules

Identified March 29, 2026More igneous →

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Physical properties

Formation & geological history

Formed from cooling lava flows where gas bubbles (vesicles) were trapped. Over millions of years, mineral-rich groundwater seeped through the porous basalt, depositing minerals like quartz, calcite, or zeolites into the voids. Common in Cenozoic or Mesozoic lava fields.

Uses & applications

Used primarily as decorative specimens for collectors, road aggregate in construction, and occasionally for lapidary work if the amygdules are large enough to be cut into 'Lake Superior Agate' or similar cabochons.

Geological facts

The term 'amygdule' comes from the Latin word 'amygdala,' meaning almond, referring to the almond-shaped mineral deposits. These rocks are key indicators for geologists of ancient volcanic gas content and hydrothermal activity.

Field identification & locations

Identify by looking for a heavy, dark, fine-grained igneous rock with distinct 'polka-stots' of lighter mineral material. Commonly found in the Lake Superior region (USA/Canada), Deccan Traps (India), and Iceland.

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

Blue Apatite

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

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral