igneous
Amygdaloidal Basalt
Vesicular Basalt with secondary mineral infilling (commonly Zeolites, Calcite, or Quartz)
Hardness: 5-6 (matrix) with variable infill hardness; Color: Dark grey, brown, or black matrix with white or clear round inclusions; Luster: Dull/Earthy (matrix) to Vitreous (inclusions); Structure: Aphanitic (fine-grained) with amygdaloidal (bubble-filling) texture; Specific Gravity: 2.8-3.0.
- Hardness
- 5-6 (matrix) with variable infill hardness
- Luster
- Dull/Earthy (matrix) to Vitreous (inclusions)
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Physical properties
Hardness: 5-6 (matrix) with variable infill hardness; Color: Dark grey, brown, or black matrix with white or clear round inclusions; Luster: Dull/Earthy (matrix) to Vitreous (inclusions); Structure: Aphanitic (fine-grained) with amygdaloidal (bubble-filling) texture; Specific Gravity: 2.8-3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava. Gases trapped in the cooling lava create bubbles (vesicles). Later, mineral-rich groundwater perculates through the rock, precipitating minerals into these voids to form 'amygdules.' These can range from recent volcanic activity to billions of years old.
Uses & applications
Used primarily as decorative stone, construction aggregate when crushed, and highly valued by mineral collectors if the amygdules contain rare zeolites or semi-precious gemstones like agate.
Geological facts
The word 'amygdule' is derived from the Greek word for 'almond,' referring to the typical shape of the mineral-filled gas bubbles. This specific specimen from the Australian Museum depicts classic zeolite infilling common in New South Wales volcanic fields.
Field identification & locations
Identify in the field by looking for a heavy, dark, fine-grained rock peppered with small, round, light-colored 'spots' or crystals. Commonly found in volcanic regions such as the Deccan Traps (India), Lake Superior region (USA), and various volcanic fields in Australia and Iceland.
