igneous
Amygdaloidal Basalt
Basalt with secondary mineral infilling
Hardness: 5-6 (Mohs); Color: Dark grey-green with pink and white inclusions; Luster: Dull to earthy; Texture: Aphanitic with vesicles and amygdules; Specific Gravity: 2.8-3.0
- Hardness
- 5-6 (Mohs)
- Color
- Dark grey-green with pink and white inclusions
- Luster
- Dull to earthy
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Physical properties
Hardness: 5-6 (Mohs); Color: Dark grey-green with pink and white inclusions; Luster: Dull to earthy; Texture: Aphanitic with vesicles and amygdules; Specific Gravity: 2.8-3.0
Formation & geological history
Formed from the cooling of mafic lava flows. The holes (vesicles) were created by gas bubbles escaping the lava. Over geological time, groundwater rich in minerals deposited substances like quartz, feldspar, or zeolites into those holes, forming 'amygdules'.
Uses & applications
Used primarily as crushed stone for road construction, railroad ballast, and high-quality aggregate. Collectors value them for the mineral variety found within the amygdules.
Geological facts
The term 'amygdule' comes from the Greek word 'amygdala', meaning almond, referring to the common shape of the mineral infillings. This process often leads to the creation of beautiful agates or zeolites inside common lava rocks.
Field identification & locations
Identify in the field by looking for a dark, heavy volcanic rock with distinct filled bubble-like spots that contrast with the matrix color. Commonly found in tectonic rift zones and volcanic provinces like the Lake Superior region or the Deccan Traps.
