Lake Superior agate

Its wide distribution and iron-rich bands of color reflect the gemstone's geologic history in Minnesota, Wisconsin, Nebraska, Iowa, Kansas, and Michigan.

More than a billion years ago, the North American continent began to split apart along plate boundaries.

Magma upwelled into iron-rich lava flows throughout the Midcontinent Rift System, including what is now the Minnesota Iron Range region.

Later, groundwater transported ferric iron, silica, and other dissolved minerals passed through the trapped gas vesicles.

These quartz-rich groundwater solutions deposited concentric bands of fine-grained quartz called chalcedony, or embedded agates.

Its crushing action and cycle of freezing and thawing at its base also freed many agates from within the lava flows and transported them, too.

The advancing glacier acted like an enormous rock tumbler, abrading, fracturing, and rough-polishing the agates.

Each band, when traced around an exposed pattern or "face," connects with itself like the walls of a fort, hence the name fortification agate.

The straight bands were produced by puddles of quartz-rich solutions that crystallized inside the gas pocket under very low fluid pressure.