The Lake Superior Agate is Minnesota's State Gem. It is a type of agate stained by iron and found on the shores of Lake Superior.
The gemstone comes in various sizes. The gas pockets in which the agates formed were primarily small, about the size of a pea. A few Lake Superior agates weigh more than 20 pounds, about the size of a bowling ball. Such giant agates are extremely rare, but no doubt others are yet to be discovered.