sedimentary

Lake Superior Agate

Silicon Dioxide (Chalcedony variety)

Hardness of 6.5 to 7 on the Mohs scale, exhibits distinct concentric banding patterns in red, brown, and yellow tones, vitreous to waxy luster, microcrystalline (cryptocrystalline) crystal structure, conchoidal fracture with no true cleavage, and a specific gravity around 2.6

Identified July 29, 2026More sedimentary →

Explore Lake Superior Agate in the encyclopedia →

Identify your own rocks.

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

App Store Google Play

Physical properties

Formation & geological history

Formed roughly 1.1 billion years ago during the Keweenawan rifting event in the Midcontinent Rift System, where mineral-rich hydrothermal fluids filled gas cavities in basaltic lava flows

Uses & applications

Popularly used as a gemstone in jewelry, lapidary art, and highly sought after by rockhounds and mineral collectors

Geological facts

Lake Superior agates are the official gemstone of the US state of Minnesota, and their characteristic red and orange banding is caused by oxidized iron within the ancient volcanic environment

Field identification & locations

Identified primarily by its bold, concentric fortification bands and rich iron-oxide coloration; commonly found in glacial drift, gravel pits, and along the shores of Lake Superior and surrounding Midwestern US states

Other sedimentary specimens

See all

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Sedimentary Sandstone

Arenite (Sedimentary Rock)

sedimentary

Fossiliferous Concretion

Septarian Concretion containing Brachiopods/Crinoids

fossil

Banded Siltstone / Mudstone Pebble

Siltstone / Argillite

sedimentary

Shale (or Mudstone/Siltstone)

Sedimentary Mudrock (SiO2, Al2O3 base with clay minerals)

sedimentary

Shale

Shale (clastic sedimentary rock)

sedimentary