igneous
Amygdaloidal Rhyolite
Amygdaloidal felsic extrusive rock
Hardness: 6-7 on Mohs scale; Color: Pinkish-brown or reddish with white/cream spots; Luster: Dull/earthy matrix with vitreous inclusions; Structure: Aphanitic with vesicles filled by secondary minerals; Cleavage: None.
- Hardness
- 6-7 on Mohs scale
- Color
- Pinkish-brown or reddish with white/cream spots
- Luster
- Dull/earthy matrix with vitreous inclusions
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pinkish-brown or reddish with white/cream spots; Luster: Dull/earthy matrix with vitreous inclusions; Structure: Aphanitic with vesicles filled by secondary minerals; Cleavage: None.
Formation & geological history
Formed from the rapid cooling of silica-rich lava. The 'spots' or amygdules are bubbles (vesicles) formed by escaping gases that later filled with secondary minerals like quartz or calcite over millions of years.
Uses & applications
Primarily used for lapidary work (polishing), decorative gravel, soapstone carving, and as educational specimens for geology students.
Geological facts
The term 'amygdule' comes from the Greek word for almond. These rocks are frequently found as water-worn pebbles in glacial till across the Great Lakes region after being transported from volcanic flows.
Field identification & locations
Identify by looking for a fine-grained reddish matrix containing 'almond-shaped' mineral pockets. Found in volcanic regions or glacial deposits. Common in the Keweenaw Peninsula and around Lake Superior.
