igneous
Basalt with Amygdaloidal Infilling
Porphyritic or Amygdaloidal Basalt
Hardness: 6-7 Mohs; Color: Dark grey, greenish-grey to black with lighter green or white inclusions; Luster: Dull to earthy; Crystal Structure: Aphanitic (fine-grained) groundmass; Cleavage: None.
- Hardness
- 6-7 Mohs
- Luster
- Dull to earthy
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Physical properties
Hardness: 6-7 Mohs; Color: Dark grey, greenish-grey to black with lighter green or white inclusions; Luster: Dull to earthy; Crystal Structure: Aphanitic (fine-grained) groundmass; Cleavage: None.
Formation & geological history
Formed from the rapid cooling of basaltic lava. The holes (vesicles) were created by gas bubbles and later filled with secondary minerals like epidote, chlorite, or quartz through hydrothermal activity.
Uses & applications
Commonly used as crushed stone for aggregate in construction, road base, and railroad ballast. Exceptionally patterned pieces are used for lapidary work or landscape decoration.
Geological facts
Many of these green-spotted basalts found in the Great Lakes region are part of the Keweenawan Midcontinent Rift System, dating back approximately 1.1 billion years.
Field identification & locations
Identify in the field by its heavy weight, dark base color, and distinct circular or irregular mineral 'spots.' Commonly found on beaches of the Olympic Peninsula or the Great Lakes.
