Quartz and chalcedony geode, Keokuk, Iowa

Description
“Geodes” means “earthlike,” yet the geodes are never perfectly spherical, or even perfectly oblate spherical, but always a little lumpy. The ones from Keokuk in southeastern Illinois are among the very best. Although origins are not agreed upon, the first stage involves forming concretions in soft mud, at shallow burial depth. Then, mineralizing groundwater solutions full of silica harden the outer shell of the concretion (producing a chalcedony shell), while the interior of the original concretion is hollowed out. Later, therefore, mineralizing solutions can ‘come along’ and deposit crystals inside the hollowed orb, commonly producing a succession of mineral/crystal layers, thus revealing the sequencing of groundwater chemistry. Some become filled completely. Some remain mostly hollow; these are the ones that can be the best, for there is room for beautiful crystals to retain their crystallographic forms.
Whereabouts

Back in the summer of 1962 I was working for Consolidation Coal Company, and spent part of my time on a drilling project in Illinois.  I took a Sunday to drive down in my oil-burning 1956 Ford to the region of Keokuk to look for geodes.  Along one back road I came across a stream, where geodes had been eroding from the bank.  I cannot recall who, if anyone, directed me there.  By the end of the afternoon, I had 3 bushels of geodes in the trunk of my car.  The best one was a tiny (2 cm diameter) gray geode, inside of which was a single pink saddle dolomite crystal on a bed of drusy quartz.  I don’t know what happened to it.  I have never had such fun collecting.  I would heft a geode, hoping that it was really light, and then give it a tap with my rock hammer.  It would split along a jagged fissure, and open up to reveal what lay inside.  It was an amazing afternoon.  One geode had needles of marcasite.  Some had tiny pyrite cubes.  Most were full of quartz and chalcedony.  The largest one I collected was 15-cm across.  I like the mystery of origin of geodes, and the fascination of what can happen during burial of sediments.

Age & Formation
Warsaw Formation of Mississippian age, ~340 Ma.