They generally are easy to see because of their shape and the texture of their outer shell. The micro-environment inside the shell is an excellent place for crystal growth. Temperature and pressure changes, as well as ev aporation, cause the mineral matter to precipitate. More solutions rich in minerals may seep into the geode later, adding to the quartz crystals or forming other minerals. In addition to the chalcedony of the outer shell, the insides of some geodes are lined with a pronounced bumpy, mammillary form of blue-gray chalcedony.
Some specimens also have excellent clear quartz crystals.
Perhaps the most fascinating geodes are those that contain petroleum, which may be under enough pressure to squirt out when the geode is broken. The rock in which these unusual geodes are found north of Nauvoo, no longer contains any significant oil. We don't know for sure. Perhaps trace amounts of some of the elements that make up the rarer minerals were present in shale layers associated with the carbonate strata. These sedimentary strata were deposited in shallow seas that covered what is now the midcontinent about 350 million years ago.