igneous
Amygdaloidal Basalt with Epidote
Amygdaloidal Basalt (Mafic volcanic rock)
Hardness (Mohs scale): 5-7. Color: Dark gray to black matrix with pistachio green (epidote) and white (zeolite/quartz/calcite) inclusions. Luster: Dull/earthy for matrix, vitreous for amygdules. Structure: Fine-grained with vesicules (gas bubbles) filled by secondary minerals.
- Luster
- Dull/earthy for matrix, vitreous for amygdules
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Physical properties
Hardness (Mohs scale): 5-7. Color: Dark gray to black matrix with pistachio green (epidote) and white (zeolite/quartz/calcite) inclusions. Luster: Dull/earthy for matrix, vitreous for amygdules. Structure: Fine-grained with vesicules (gas bubbles) filled by secondary minerals.
Formation & geological history
Formed from the rapid cooling of low-viscosity lava. Gas bubbles (vesicles) trapped during cooling were later filled with secondary minerals through hydrothermal activity or precipitation from groundwater. Often associated with Precambrian volcanic sequences like the Midcontinent Rift.
Uses & applications
Primarily used as road aggregate or construction material in bulk. Specific specimens with high epidote or agate filling are popular with mineral collectors and lapidaries for cabochons.
Geological facts
This rock type is famous for containing Lake Superior Agates and native copper in the Keweenaw Peninsula region of Michigan. The green color is caused by epidote, a calcium aluminum iron silicate mineral.
Field identification & locations
Identify in the field by looking for a heavy, dark, fine-grained rock with small 'almond-shaped' colorful spots. It is commonly found in basaltic lava flow regions, particularly old glacial till or near riverbeds in volcanic zones.
