igneous
Greenstone / Basalt with Amygdule
Amygdaloidal Basalt (altered to Greenstone)
Hardness: 5.5-6.5 (Mohs); Color: Dark greenish-gray with light green/white inclusions; Luster: Dull to earthy; Texture: Aphanitic (fine-grained) with amygdules (void fillings); Cleavage: None visible.
- Hardness
- 5
- Color
- Dark greenish-gray with light green/white inclusions
- Luster
- Dull to earthy
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Physical properties
Hardness: 5.5-6.5 (Mohs); Color: Dark greenish-gray with light green/white inclusions; Luster: Dull to earthy; Texture: Aphanitic (fine-grained) with amygdules (void fillings); Cleavage: None visible.
Formation & geological history
Formed from cooling of mafic lava flows where gas bubbles (vesicles) were trapped. Over geological time, secondary minerals like epidote or chlorite filled these holes (creating amygdules) and slightly metamorphosed the rock into greenstone.
Uses & applications
Commonly used as crushed stone in construction, road base, and occasionally as decorative river rock or for lapidary work if the inclusions are attractive.
Geological facts
The term 'greenstone' is a general field term for dark, greenish altered igneous rocks. The 'spots' seen in this specimen are mineral-filled gas bubbles from the original volcanic eruption.
Field identification & locations
Identify by the dark green hue and the presence of rounded, lighter-colored mineral 'knots' or fillings. Common in volcanic terrains and glaciated riverbeds in the Great Lakes region or Pacific Northwest.
