igneous
Amygdaloidal Basalt
Basalt with secondary mineral infilling (commonly SiO2 or Zeolites)
Hardness: 5-6 (matrix); Color: Dark grey to brown with lighter amygdules; Luster: Dull to earthy; Structure: Vesicular/fine-grained; Specific Gravity: 2.8-3.0
- Hardness
- 5-6 (matrix)
- Color
- Dark grey to brown with lighter amygdules
- Luster
- Dull to earthy
Identified More igneous →
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Physical properties
Hardness: 5-6 (matrix); Color: Dark grey to brown with lighter amygdules; Luster: Dull to earthy; Structure: Vesicular/fine-grained; Specific Gravity: 2.8-3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava. The holes (vesicles) were created by gas bubbles, which later filled with secondary minerals through groundwater precipitation.
Uses & applications
Used as crushed stone for aggregate in construction, road base, and occasionally as a decorative garden stone or for beginner rock tumbling.
Geological facts
Many of the famous Lake Superior agates were originally formed inside the cavities of this type of basaltic rock over a billion years ago during the Midcontinent Rift.
Field identification & locations
Identified by gas-bubble shapes (amygdules) filled with a different colored mineral against a dark, fine-grained background. Found in volcanic regions like the Pacific Northwest or the Great Lakes.
