Igneous
Amygdaloidal Basalt
Amygdaloidal Basalt (Mafic volcanic rock including secondary mineral infills)
Hardness: 5-7 (groundmass), Color: dark grey to black matrix with white, green, or pink circular spots, Luster: dull to earthy matrix with vitreous inclusions, Structure: Vesicular/Porphyritic, Cleavage: None
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Physical properties
Hardness: 5-7 (groundmass), Color: dark grey to black matrix with white, green, or pink circular spots, Luster: dull to earthy matrix with vitreous inclusions, Structure: Vesicular/Porphyritic, Cleavage: None
Formation & geological history
Formed from the cooling of magnesium-rich and iron-rich lava. Gases trapped in the lava create bubbles (vesicles), which are later filled by secondary minerals like quartz, calcite, or zeolites precipitated from groundwater.
Uses & applications
Used primarily in construction as crushed stone or aggregate. Historically used for carving. Attractive specimens are used for educational purposes and lapidary cutting.
Geological facts
The word 'Amygdaloidal' comes from 'amygdala', Latin for almond, referring to the almond-shaped mineral inclusions. These rocks are primary sources for high-quality agate and zeolites.
Field identification & locations
Identify by looking for a dark, dense rock base filled with distinct, rounded 'spots' or 'pebbles' of a different color. Common in volcanic regions like the Lake Superior basin or the Deccan Traps.
