igneous
Amygdaloidal Basalt
Basalt with secondary mineral infillings (SiO2, (Ca,Na)2-3(Mg,Fe,Al)5Si8O22(OH)2)
Hardness: 5-7 (Mohs). Color: Dark gray to black matrix with multicolored inclusions (green, pink, white). Luster: Dull to vitreous. Structure: Aphanitic with vesicles filled by secondary minerals. Specific Gravity: 2.8-3.0.
- Hardness
- 5-7 (Mohs)
- Luster
- Dull to vitreous
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Physical properties
Hardness: 5-7 (Mohs). Color: Dark gray to black matrix with multicolored inclusions (green, pink, white). Luster: Dull to vitreous. Structure: Aphanitic with vesicles filled by secondary minerals. Specific Gravity: 2.8-3.0.
Formation & geological history
Formed from cooling lava with trapped gas bubbles (vesicles). Over millions of years, mineral-rich groundwater deposits substances like epidote or quartz into these voids. Often associated with Precambrian volcanic sequences.
Uses & applications
Primarily used as a decorative stone, lapidary material for jewelry (cabochons and pendants), and as a collector's specimen. Its toughness also makes it suitable for ornamental landscaping.
Geological facts
The term 'amygdule' comes from the Latin word 'amygdala', meaning almond, referring to the almond-shaped mineral deposits. This specific variety with green and pink inclusions is often found in the Lake Superior region of North America.
Field identification & locations
Identify by the dark, fine-grained volcanic matrix containing distinct, rounded 'pockets' of different colored minerals. Commonly found in basaltic lava flow fields, especially in the Keweenaw Peninsula, Michigan.
