igneous

Amygdaloidal Rhyolite

Amygdaloidal felsic extrusive rock

Hardness: 6-7 on Mohs scale; Color: Pinkish-brown or reddish with white/cream spots; Luster: Dull/earthy matrix with vitreous inclusions; Structure: Aphanitic with vesicles filled by secondary minerals; Cleavage: None.

Hardness: 6-7 on Mohs scale
Color: Pinkish-brown or reddish with white/cream spots
Luster: Dull/earthy matrix with vitreous inclusions

Identified May 16, 2026More igneous →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed from the rapid cooling of silica-rich lava. The 'spots' or amygdules are bubbles (vesicles) formed by escaping gases that later filled with secondary minerals like quartz or calcite over millions of years.

Uses & applications

Primarily used for lapidary work (polishing), decorative gravel, soapstone carving, and as educational specimens for geology students.

Geological facts

The term 'amygdule' comes from the Greek word for almond. These rocks are frequently found as water-worn pebbles in glacial till across the Great Lakes region after being transported from volcanic flows.

Field identification & locations

Identify by looking for a fine-grained reddish matrix containing 'almond-shaped' mineral pockets. Found in volcanic regions or glacial deposits. Common in the Keweenaw Peninsula and around Lake Superior.

Other igneous specimens

See all

Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Magnetite

Magnetite (Fe3O4)

mineral