igneous
Amygdaloidal Basalt
Amygdaloidal Basalt (Mafic volcanic rock with secondary mineral fillings)
Hardness: 5-7 (groundmass) with softer fillings; Color: Dark grey to reddish-brown matrix with white inclusions; Luster: Dull/Earthly matrix, vitreous/pearly inclusions; Texture: Vesicular/Porphyritic; Gravity: 2.8-3.0.
- Hardness
- 5-7 (groundmass) with softer fillings
- Color
- Dark grey to reddish-brown matrix with white inclusions
- Luster
- Dull/Earthly matrix, vitreous/pearly inclusions
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Physical properties
Hardness: 5-7 (groundmass) with softer fillings; Color: Dark grey to reddish-brown matrix with white inclusions; Luster: Dull/Earthly matrix, vitreous/pearly inclusions; Texture: Vesicular/Porphyritic; Gravity: 2.8-3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava. The white 'spots' (amygdules) form when gas bubbles (vesicles) in the cooling lava are later filled with secondary minerals like quartz, calcite, or zeolites through groundwater infiltration.
Uses & applications
Used primarily as crushed stone for construction, road base, and railroad ballast. Decorative specimens are popular among amateur collectors and educators.
Geological facts
The word 'amygdule' comes from the Latin 'amygdala', meaning almond, referring to the almond-shaped mineral deposits. This rock is a common host for semi-precious gemstones like agates and copper in regions like Lake Superior.
Field identification & locations
Identify by looking for a dark, dense, fine-grained volcanic matrix with distinct, rounded light-colored mineral 'pockets'. Commonly found in volcanic fields, oceanic islands, and ancient rift zones like the Keweenaw Peninsula.
