igneous
Amygdaloidal Rhyolite
Amygdaloidal Rhyolite (Felsic volcanic rock)
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
- Hardness
- 6-7 on Mohs scale
- Color
- Reddish-brown, maroon, or dark purple with pink/white spots
- Luster
- Dull to sub-vitreous
Identified More igneous →
Identify your own rocks.
Get a report just like this from any photo, free.
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.
