Photo from: http://rocktumbler.com/rough/lake-superior-agate.shtml

Lake Superior Agates are a type of agate found near Lake Superior.  They were carried across Minnesota by glaciers 10,000 years ago and are so common there that they have been Minnesota’s official state gemstone since 1969.  Wisconsin and Michigan have some, too.  Lake Superior agates are known for their red color which comes from iron oxide in the surrounding area.  Most Lake Superior agates are banded agates, but a few are eye agates, some are “waterwashed” agates (called so because they have been naturally polished by the water on the beach, like sea glass), and, rarest of all, some of them weigh over 2 pounds.

I should also point out that if you ever go looking for agates, the ones in the field won’t be as colorful and they are easy to mistake for other rocks like granite or basalt.  Most of the pictures of agates are of nice pretty polished slabs or cabochons, and that tends to give people unrealistic expectations.  That’s why the photo shows Lake Superior agates before and after tumbling.

The Minnesota Department of Natural Resources was invaluable in writing this post.  More information here: http://www.dnr.state.mn.us/education/geology/digging/agate.html