Amygdaloidal Basalt

Physical Properties

Hardness: 6-7 (basalt matrix); Color: Dark grey to reddish-brown matrix with white, pink, or orange 'amygdules'; Luster: Dull to earthy; Texture: Vesicular (originally) now filled; Crystal structure: Afanitic (fine-grained).

Formation & Geological History

Formed from the cooling of lava flows where gas bubbles (vesicles) were trapped. Over millions of years, mineral-rich groundwater seeped through the rock, depositing minerals like quartz, feldspar, or zeolites into the bubbles.

Uses & Applications

Used primarily as decorative garden stones, in educational geological collections, or occasionally tumbled for lapidary work if the mineral fillings are colorful like agate.

Geological Facts

The term 'amygdule' comes from the Latin word 'amygdala', meaning almond, because the mineral-filled gas bubbles often resemble almond nut shapes. These rocks are very common in the Lake Superior region of North America.

Field Identification & Locations

Identify in the field by looking for a dense, dark volcanic rock peppered with distinct, rounded spots of a different mineral color. Often found in ancient volcanic rift zones and along glacial gravel deposits.

Identified on: 4/22/2026

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