Some say Lake Superior agates are the oldest in the world. However that may be, hobbyists like to search for them, and anyone can appreciate the remarkable colors and intricate patterns of the polished stones. In 1969 the Minnesota Legislature declared the Lake Superior agate to be the state's official gemstone. Moose Lake, Minnesota has dubbed itself the "agate capital of the world" and Agate Days is celebrated there each year in July.
General description: Lake Superior agates are generally shaped as irregular spheres. They are made up of quartz, often reddened by iron and deposited in layers to create concentric circles that look like the rings on the cross section of a tree.
Size: Lake Superior agates range from about the size of a pea to up to more than 20 pounds.
Color: Red, orange, and yellow, all caused by iron, are the main colors in Lake Superior agates.
The history of Lake Superior agates traces back to about a billion years ago. The North American continent began to split, creating a large rift valley, and lava welled up in the area of what is now Lake Superior. Bubbles of air were trapped in the lava (similar to the way bubbles of air appear in a pan of water before it begins to boil.) After the lava cooled, water seeped into the holes created by the bubbles and deposited iron, quartz, and other minerals in layers, creating agates. As the surrounding volcanic rock was worn away by erosion or the scouring action of glaciers, agates were released from the lava and moved to other places.
Rock collectors classify Lake Superior agates according to their appearance. For example, agates that have a cross section showing circles or other shapes repeated in many layers are called fortification agates. The name comes from the fact that the shapes look like the walls of a fortress. The eye agate has circles that look a little bit like eyes on its surface. Moss agates have tree-branch-shaped bits of minerals trapped in them.
Despite their name, Lake Superior agates can be found throughout much of Minnesota. That's because 10,000 years ago, glaciers carried them far from their origin in the Lake Superior region. People often look for them on the banks or at the mouths of rivers, in gravel pits, or in other places where pebbles and gravel abound.
Agate collecting is an enjoyable hobby for many people. Collectors value rocks for their size and unusual markings. Large, exceptionally beautiful agates may be worth hundreds of dollars. A rock and mineral show is a good place to see many different agates.