Amygdaloidal Basalt

Hardness of 6 on the Mohs scale; color ranges from dark gray to black with lighter mineral infills; dull to sub-withous luster; aphanitic to porphyritic crystal structure with rounded amygdules; specific gravity around 2.8 to 3.0.

Formed from mafic volcanic lava flows where gas bubbles (vesicles) formed cavities that were later filled with secondary minerals such as quartz, calcite, or zeolites through hydrothermal or groundwater solutions.

Used in crushed stone for construction, aggregate in concrete, railroad ballast, and sometimes collected or cut as decorative lapidary material when filled with attractive minerals.

The characteristic spots are called amygdules, which derive from the Greek word for almond due to their typical shape. When these filled vesicles weather out of the softer host basalt, they can form pebble-like gemstones.

Identified in the field by a dark, fine-grained volcanic rock matrix peppered with distinct rounded spots or mineral-filled cavities. Commonly found in regions with ancient or active flood basalt provinces like the Keweenaw Peninsula of Michigan, the Columbia River Plateau, and Iceland.