sedimentary

Lake Superior Agate

Silicon dioxide (chalcedony variety quartz)

Hardness of 7 on the Mohs scale; displays concentric banding in shades of red, orange, and brown due to iron oxide staining; waxy to vitreous luster; microcrystalline (cryptocrystalline) crystal structure; no cleavage; specific gravity of approximately 2.60 to 2.65.

Identified August 4, 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 Rift Zone event. Mineral-rich hydrothermal fluids deposited silica in gas cavities within basaltic lava flows over long periods.

Uses & applications

Prized by rockhounds and lapidaries for polishing, tumbling, and crafting into jewelry and decorative items.

Geological facts

Lake Superior agates are the official state gemstone of Minnesota and are famous for their rich red and orange banding caused by oxidized iron, which reflects the region's ancient volcanic and glacial history.

Field identification & locations

Easily identified in the field by its characteristic concentric fortification banding, waxy luster, and reddish-brown iron-stained exterior. Commonly found in glacial drift, gravel pits, and beaches around the Lake Superior region.

Other sedimentary specimens

See all

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Quartz pebble

Silicon dioxide (SiO2)

sedimentary

Fossiliferous Concretion

Septarian Concretion containing Brachiopods/Crinoids

fossil

Generic Pebble / Rock Fragment

Lithic fragment / Weathered rock

sedimentary

Sedimentary Sandstone

Arenite (Sedimentary Rock)

sedimentary

Banded Siltstone / Mudstone Pebble

Siltstone / Argillite

sedimentary