igneous
Amygdaloidal Basalt
Amygdaloidal Mafic Volcanic Rock
Hardness: 6-7 (Mohs); Color: Dark grey to reddish-brown matrix with light white/green mineral fillings; Luster: Dull to earthy (matrix), Vitreous (fillings); Crystal Structure: Aphanitic (fine-grained) with secondary mineral crystal clusters; Specific Gravity: 2.8–3.0.
- Hardness
- 6-7 (Mohs)
- Luster
- Dull to earthy (matrix), Vitreous (fillings)
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Physical properties
Hardness: 6-7 (Mohs); Color: Dark grey to reddish-brown matrix with light white/green mineral fillings; Luster: Dull to earthy (matrix), Vitreous (fillings); Crystal Structure: Aphanitic (fine-grained) with secondary mineral crystal clusters; Specific Gravity: 2.8–3.0.
Formation & geological history
Formed from the cooling of gas-rich lava. As the lava cools, gas bubbles (vesicles) are trapped; later, mineral-rich groundwater flows through these holes, depositing minerals like quartz, calcite, or epidote to fill them. Often Proterozoic in age (approx. 1 billion years old in the Great Lakes region).
Uses & applications
Primary use as decorative landscaping stones and beach-combing collectibles. Occasionally used in construction aggregate or polished for lapidary work and unique jewelry cabochons.
Geological facts
The term 'amygdule' comes from the Greek word 'amygdalo', meaning 'almond', referring to the almond shape of the mineral fillings. These rocks are most famous for being the primary host rock of native copper in Michigan's Keweenaw Peninsula.
Field identification & locations
Identified in the field by its 'polka-dot' appearance caused by light-colored mineral circles against a darker, weathered volcanic crust. Commonly found on beaches of Lake Superior or mixed into glacial till throughout the American Midwest.
