“ ... Jacob's Quarry (Hamilton, Illinois) is one of several geode-producing sites around Keokuk, Iowa and I like coming to the quarry mainly because the geodes are of right size for me (not bigger than softballs) to sell for geode-cracking purposes. I did not expect to find any agates there and I almost threw the solid nodules away. When I broke one that turned to contain the white agate bands, I gathered the solid nodules that I threw away to be cut. Few nodules will reveal the "White Ringers" a nickname given to the ring-like white bands of agates within the quartz nodules by the local collectors and apparently, they have some value among the geode-collectors.
The nodules originally were the form of crinoid head and other fossils before they had been expanded by crystallizing gypsum and other evaporate within the soft limestone (or limey mudstone) to be later dissolved. Quartz, calcite, dolomite, barite, marcasite and sometimes agates will fill in the gaps left by dissolved evaporate minerals. Hollow and chalcedony lined (also agate) geodes can be also found there.
The quarry is not the only place where agates can be found. The geode-bearing Warsaw Formation covering a large area around Keokuk, Iowa (including Canton, Missouri and possibly Vinton, Iowa) will sometimes produce large white to gray banded agate nodules similar to the Coldwater Agates from Vinton, Iowa...”
written by Jeffrey Anderson on his website dwarves earth-treasures.