✦ New find ·
Agate
Lake Superior agate
- What it is
- Banded chalcedony, a very fine-grained quartz. Its tight, angular bands look like the walls of a fort, so collectors call the pattern fortification banding.
- How it forms
- About 1.1 billion years ago, lava flows along the Midcontinent Rift trapped gas bubbles. Silica-rich water slowly filled them, layer by layer. Iron gives the red, orange and yellow.
- Where it’s found
- Beaches and glacial gravels around Lake Superior, in Minnesota, Wisconsin and Michigan. Glaciers carried many far from where they formed. It is Minnesota's state gemstone.
Photos: Rough (found in northeast Iowa): IowaAgateMan, CC0. Cut: Lech Darski, CC BY-SA 4.0.