Minnesota's state rock is the Lake Superior agate--so named
because it is found predominantly along the coast of Lake Superior.
Agates form in cavities in basalt. As mineral-rich water circulates
through the cavities, silica (SiO2), or quartz,
is deposited in layers along cavity walls. Eventually, the cavities
completely fill with this banded variety of quartz. The color variations
are due to slight mineral impurities in the water. Iron, for example,
causes much of the red and orange color seen in Lake Superior agates.