igneous

Amygdaloidal Rhyolite

Rhyolite (felsic volcanic rock) with amygdules

Hardness: 6-7 on Mohs scale. Color: Reddish-brown with white amygdules. Luster: Dull/earthy matrix with vitreous inclusions. Crystal structure: Aphanitic (fine-grained) with secondary mineral infilling. Specific gravity: 2.4–2.6.

Hardness: 6-7 on Mohs scale
Color: Reddish-brown with white amygdules
Luster: Dull/earthy matrix with vitreous inclusions

Identified March 29, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of silica-rich lava. The 'holes' (vesicles) were gas bubbles trapped in the cooling lava; these were later filled by secondary minerals like quartz, calcite, or zeolites through hydrothermal activity, creating 'amygdules'. These specimens are often found in Precambrian or Cenozoic volcanic regions.

Uses & applications

Used primarily as decorative landscaping gravel, road aggregate, and occasionally tumbled for lapidary use by collectors.

Geological facts

Rhyolite is the volcanic equivalent of granite. The red color is typically due to high iron oxide content within the silicate matrix. These rounded specimens are often found in glaciated regions or riverbeds where they have been naturally tumbled.

Field identification & locations

Identify by its reddish/purple fine-grained matrix and distinct white orbital spots. Commonly found in the Lake Superior region (USA), Scotland, and New Zealand. Look for the contrast between the hard volcanic glass-like base and the mineral-filled pockets.

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