igneous
Amygdaloidal Basalt
Basalt (Igneous Volcanic Rock with secondary mineral infilling)
Hardness: 6-7 (matrix); Color: dark grey to brownish-purple; Luster: dull to earthy matrix with potentially vitreous fillings; Texture: porphyritic or vesicular; contains amygdules (mineral-filled gas bubbles).
- Hardness
- 6-7 (matrix)
- Color
- dark grey to brownish-purple
- Luster
- dull to earthy matrix with potentially vitreous fillings
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Physical properties
Hardness: 6-7 (matrix); Color: dark grey to brownish-purple; Luster: dull to earthy matrix with potentially vitreous fillings; Texture: porphyritic or vesicular; contains amygdules (mineral-filled gas bubbles).
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava. The holes (vesicles) were formed by expanding gas bubbles, which were later filled with secondary minerals like quartz, calcite, or zeolites through hydrothermal activity.
Uses & applications
Used primarily as crushed stone for aggregate in construction, road base, and railroad ballast. Exceptionally attractive pieces are used as aquarium stones or by mineral collectors.
Geological facts
Many of the world's most famous copper deposits, like those in the Keweenaw Peninsula of Michigan, are found within the amygdules of ancient basalt flows.
Field identification & locations
Identify by looking for small 'pockmarks' or round colored mineral inclusions in a dark, fine-grained volcanic rock. Often found in areas with historic volcanic activity or as glacial till in riverbeds.
