igneous
Basalt-Amugydule (Amygdaloidal Basalt)
Aphanitic Mafic Igneous Rock with Vesicle Infillings
Hardness: 5-6 (the host basalt), Color: dark grey to black matrix with light-colored mineral pockets, Luster: dull for the matrix, vitreous for infusions, Crystal structure: microcrystalline, Cleavage: none.
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Physical properties
Hardness: 5-6 (the host basalt), Color: dark grey to black matrix with light-colored mineral pockets, Luster: dull for the matrix, vitreous for infusions, Crystal structure: microcrystalline, Cleavage: none.
Formation & geological history
Formed from cooling lava flows where gas bubbles (vesicles) were trapped. Over geological time, groundwater filled these holes with minerals like quartz, calcite, or zeolites. Can date from various volcanic eras across hundreds of millions of years.
Uses & applications
Used in construction as crushed stone or aggregate. High-quality specimens with colorful agate or amethyst infillings are used for jewelry or as decorative collection pieces.
Geological facts
The holes are called 'vesicles' when empty and 'amygdules' (from the Greek for almond) when they are filled with secondary minerals.
Field identification & locations
Identify by the dark, heavy matrix with distinct 'spots' of different minerals. Commonly found in old volcanic regions like the Lake Superior area, Oregon, or Ireland.
