igneous
Amygdaloidal Basalt
Amygdaloidal Basalt (with Epidote and Feldspar/Zeolite infill)
AI-generated identification · may be incorrectHardness: 5-6 (matrix); Color: Dark grey to black matrix with green (epidote) and pink/white (feldspar or zeolites) almond-shaped inclusions; Luster: Dull/Earthy; Structure: Vesicular/Amygdaloidal; Specific Gravity: 2.8-3.0
- Hardness
- 5-6 (matrix)
- Luster
- Dull/Earthy
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Physical properties
Hardness: 5-6 (matrix); Color: Dark grey to black matrix with green (epidote) and pink/white (feldspar or zeolites) almond-shaped inclusions; Luster: Dull/Earthy; Structure: Vesicular/Amygdaloidal; Specific Gravity: 2.8-3.0
Formation & geological history
Formed from the cooling of lava flows where gas bubbles (vesicles) were trapped. Over millions of years, mineral-rich groundwater circulated through the rock, depositing minerals like epidote and K-feldspar into the voids. Common in the Lake Superior region (USA), dating to the Mesoproterozoic era (approx. 1.1 billion years ago).
Uses & applications
Used primarily as decorative path stones, for lapidary work (polishing), and as a study specimen for volcanic processes. Basalt itself is used in crushed stone for construction.
Geological facts
The term 'amygdule' comes from the Greek word for 'almond,' referring to the shape of the mineral-filled gas bubbles. These rocks are often associated with native copper deposits in regions like Michigan's Keweenaw Peninsula.
Field identification & locations
Identify by looking for a dark, fine-grained volcanic matrix containing distinct, rounded, or oval pods of different colored minerals. They are frequently found as water-worn cobbles on beaches in the Great Lakes region or in glacial till.
