Amygdaloidal Basalt with Agate/Zeolite Amygdules

Hardness varies from 3-4 for some secondary minerals like calcite to 6.5-7 for quartz/agate infills. Color is typically brownish-gray with contrasting reddish (often ferruginous or jasper) and greenish (celadonite or epidote) rounded mineral inclusions. Luster ranges from dull in the basalt matrix to vitreous or waxy in the amygdules. Cryptocrystalline or microcrystalline structure.

Formed when gas bubbles (vesicles) in cooling lava flows are later filled by groundwater rich in dissolved minerals, precipitating secondary minerals over millions of years.

Commonly collected by rockhounds, used for ornamental purposes, lapidary work, and in geological education to study magmatic and hydrothermal processes.

The distinct rounded spots are called amygdules or amygdaloids. When the host basalt weathers away, these harder secondary minerals often remain as resistant pebbles known as beach agates or thunderhead minerals.

Easily identified in the field by a dark or weathered grayish-brown matrix dotted with contrasting rounded spots of different colors (reds, greens, or whites). Commonly found in ancient volcanic provinces, lake superior regions, and basalt flows worldwide.