But once the humble geode, declared Iowa's state rock in 1967, is cracked opened, packed inside its gray interior is a sparkling world of wonder. Most scientists agree that there's no satisfactory answer to how these rocks developed, but here are some key points of interest. How'd they get there Over 340 million years ago during the Mississippian period, mud deposited in an epi-continental shallow sea that subsequently formed shale and limestone. Some geoogists think geodes may indicate shorelines of those long-ago warm water seas.
Colors range from clear to white to blue, pink, gray, yellow, or orange.
Some geodes are hollow, some are not. The ones that aren't hollow are filled solid with glittering crystals. If you're experienced, you can judge the inside by the weight of the stone. The ancient Greek philospher Pliny the Elder thought clear quartz crystals were water that had frozen at such low temperatures it would never thaw. Where to look While they can be found all around, some of the most beautiful geodes lie within an 80-mile radius around Keokuk including Lee, Henry, and Van Buren counties.