igneous
Amygdaloidal Basalt
Amygdaloidal Basalt (Mafic volcanic rock)
Hardness: 5-6 (Mohs scale); Color: Dark grey, black, or reddish-brown matrix with white, clear, or green infillings; Luster: Dull matrix with vitreous (glassy) or pearly vugs; Structure: Vesicular/Amygdaloidal (pitted with filled holes); Specific Gravity: 2.8 - 3.0.
- Hardness
- 5-6 (Mohs scale)
- Luster
- Dull matrix with vitreous (glassy) or pearly vugs
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Dark grey, black, or reddish-brown matrix with white, clear, or green infillings; Luster: Dull matrix with vitreous (glassy) or pearly vugs; Structure: Vesicular/Amygdaloidal (pitted with filled holes); Specific Gravity: 2.8 - 3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava (basalt). During cooling, gas bubbles leave cavities (vesicles). Over millions of years, groundwater deposits minerals like quartz, calcite, or zeolites into these holes, turning them into 'amygdules.'
Uses & applications
Used primarily in construction as crushed stone or aggregate. High-quality specimens with semi-precious infillings (like agate) are used for jewelry, lapidary work, and geological collections.
Geological facts
The word 'amygdule' comes from the Greek word for 'almond,' describing the shape of the mineral infillings. This rock is a primary source for Native American copper artifacts in the Lake Superior region, where native copper often fills the cavities.
Field identification & locations
Identify by looking for a heavy, dark, fine-grained rock with distinct pits or 'pockets' containing different colored minerals. Commonly found in volcanic regions such as the Deccan Traps (India), the Giant’s Causeway (Ireland), and the Lake Superior North Shore (USA/Canada).
