igneous
Amygdaloidal Basalt
Amygdaloidal Basalt (extrusive volcanic rock with mineral inclusions)
Hardness: 6-7 (groundmass), 3-7 (vesicle fillings); Color: Dark brown to dark gray matrix with light-colored (white/green/pink) circular spots; Luster: Dull to earthy matrix, vitreous to silky fillings; Structure: Porphyritic or vesicular; Specific gravity: 2.8-3.0.
- Hardness
- 6-7 (groundmass), 3-7 (vesicle fillings)
- Luster
- Dull to earthy matrix, vitreous to silky fillings
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Physical properties
Hardness: 6-7 (groundmass), 3-7 (vesicle fillings); Color: Dark brown to dark gray matrix with light-colored (white/green/pink) circular spots; Luster: Dull to earthy matrix, vitreous to silky fillings; Structure: Porphyritic or vesicular; Specific gravity: 2.8-3.0.
Formation & geological history
Formed from the cooling of gas-rich lava. As the lava solidifies, gas bubbles are trapped, creating cavities (vesicles). Later, mineral-rich groundwater flows through, depositing minerals like quartz, calcite, or zeolites into the holes. Most specimens are from the Cenozoic or Mesozoic eras.
Uses & applications
Commonly used as lapidary material for polishing and making jewelry (cabochons), as well as decorative garden stones and road aggregate in areas where it is abundant.
Geological facts
The name 'Amygdaloid' comes from the Greek word for 'almond' because the mineral-filled cavities often resemble almond seeds in shape and size. These rocks are frequently indicators of ancient volcanic activity near water sources.
Field identification & locations
Identify by looking for a dark, fine-grained rock containing distinct rounded 'spots' of different minerals. Commonly found in basaltic flow regions like Lake Superior (USA), the Deccan Traps (India), and Iceland. To field test, check if the white spots react with acid (calcite) or are scratch-resistant (quartz).
