Amygdaloidal Rhyolite

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Reddish-brown, maroon, or dark purple with pink/white spots; Luster: Dull to sub-vitreous; Texture: Fine-grained/aphanitic with amygdules (void-filling minerals); Specific Gravity: 2.4-2.6

Formation & Geological History

Formed from the cooling of silica-rich (felsic) lava. The 'spots' are amygdules—gas bubbles that were later filled with secondary minerals like quartz, calcite, or zeolites during late-stage hydrothermal activity.

Uses & Applications

Used primarily in landscaping, road construction as aggregate, and by lapidary hobbyists for polishing into 'Lake Superior' style reach stones or cabochons.

Geological Facts

This specific variety is highly characteristic of the Midcontinent Rift System in North America, particularly around the Lake Superior region. Many collectors find these as smooth, water-worn pebbles on beaches.

Field Identification & Locations

Identify by looking for a fine-grained, hard, reddish matrix containing small round or oval 'eyes' of a different color. Common in glacial till and Great Lakes shorelines. It is non-reactive to acid unless the amygdules are calcite.

Identified on: 4/30/2026

Mode: Standard