igneous

Amygdaloidal Basalt with Quartz/Zeolite Infill

Amygdaloidal Basalt (extrusive volcanic rock with SiO2 or CaAl2Si3O10·3H2O infill)

Hardness: 5-6 (matrix), 7 (quartz fillings); Color: Dark brown/grey matrix with clear/white or translucent crystals; Luster: Dull/earthy matrix with vitreous (glassy) fillings; Structure: Vesicular/Amygdaloidal.

Hardness: 5-6 (matrix), 7 (quartz fillings)
Luster: Dull/earthy matrix with vitreous (glassy) fillings

Identified March 28, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava. Gas bubbles (vesicles) trapped during cooling were later filled by secondary minerals like quartz, calcite, or zeolites through hydrothermal groundwater precipitation.

Uses & applications

Used primarily as crushed stone for aggregate in construction and road beds. Higher quality specimens with large crystal vugs are popular for mineral collecting and teaching tools in geology.

Geological facts

This specimen represents a 'fossilized' gas bubble. The holes are called vesicles; once they are filled with minerals, they are called amygdules, derived from the Greek word 'amygdala' meaning almond.

Field identification & locations

Identify by looking for a heavy, dark, fine-grained rock featuring small holes or 'pockets' containing contrasting crystals. Frequently found in the Lake Superior region (USA), Deccan Traps (India), and Iceland. Collectors should look for basalt flows near tectonic plate boundaries.

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Dark Igneous Rock (Likely Basalt or Diabase)

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Igneous

Magnetite

Iron(II,III) oxide (Fe3O4)

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Magnetite

Magnetite (Fe3O4)

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