Amygdaloidal Basalt

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Dark grey to reddish-brown groundmass with white, green, or translucent 'almond-shaped' inclusions; Luster: Dull/earthy matrix with vitreous inclusions; Structure: Aphanitic (fine-grained) with secondary mineral filling of vesicles.

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava. The 'amygdules' (the white/green spots) were originally gas bubbles (vesicles) trapped in the cooling lava. Later, mineral-rich groundwater flowed through the rock, depositing secondary minerals like quartz, calcite, zeolites, or epidote into these cavities.

Uses & Applications

Used primarily as decorative specimens for collectors, crushed stone for construction, and occasionally for lapidary work (cabochons) if the mineral inclusions are particularly colorful or hard.

Geological Facts

This rock is famous in the Lake Superior region of North America, where the amygdules are often filled with native copper, agates, or chlorite. These specimens are often billions of years old, dating back to the Midcontinent Rift system.

Field Identification & Locations

Identify in the field by looking for a heavy, dark, fine-grained rock with distinct light-colored spots or 'blobs' that look like they were poured into holes. Commonly found in volcanic belts, mountain ranges, and pebble beaches of volcanic lakes.

Identified on: 4/25/2026

Mode: Standard