aka minnesota agates.

these Agates and maybe other agates as well, are the next evolution of Thundereggs. the formation of this agate as lava flows which solidify as Ryolite with gas pockets within. These pockets over millions of years filled with agate.... at this point you have a Thunderegg... next over more millions of years erosion takes its course and disolves the Ryolite outer shell leaving only the agate. This agate in turn slowly got tumbled by the waters of Lake Superior forming smooth rounded shapes in most cases.

there are a few mines near the Thunder Bay/Northwestern Ontario area which has deposits of it.