igneous
Amygdaloidal Basalt with Agate/Chalcedony Infills
Amygdaloidal Basalt (Composition: SiO2 and (Mg,Fe)2SiO4 in matrix; SiO2 in amygdule)
Hardness: 6-7 (Mohs). Color: Dark grey to black matrix with translucent green/grey infills. Luster: Dull/earthy for matrix, waxy to vitreous for mineral infill. Structure: Vesicular/aphanitic. Cleavage: None.
- Hardness
- 6-7 (Mohs)
- Luster
- Dull/earthy for matrix, waxy to vitreous for mineral infill
Identified More igneous →
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Physical properties
Hardness: 6-7 (Mohs). Color: Dark grey to black matrix with translucent green/grey infills. Luster: Dull/earthy for matrix, waxy to vitreous for mineral infill. Structure: Vesicular/aphanitic. Cleavage: None.
Formation & geological history
Formed from extrusive volcanic lava flows. As gas bubbles were trapped in cooling lava, they created vesicles. Over millions of years (often Cenozoic or older), mineral-rich groundwater deposited silica (chalcedony) inside these voids.
Uses & applications
Basalt is used heavily in construction (crushed stone). The mineral infills, if high quality, are used by lapidaries for tumbling, cabochons, or as decorative display specimens.
Geological facts
The term 'amygdule' comes from the Greek word for almond, referring to the almond-shaped mineral deposits. This specimen shows a transition where a gas bubble became a small geode or solid agate node.
Field identification & locations
Identify by looking for a heavy, dark, fine-grained rock with round or oval cavities filled with a different, harder crystal or waxy mineral. Common in the Pacific Northwest (USA), Lake Superior region, and India.
