The Keokuk geode beds

Grey cannonballs that split open to crystal

Within about 35 miles of Keokuk, in Iowa's southeastern corner, lies one of the most productive and famous geode grounds anywhere, and rock collectors simply call what comes out of it Keokuk geodes. Their reputation is worldwide, and in 1967 the Iowa General Assembly made the geode the official State Rock.

From outside they are nothing much: drab, rounded, lumpy or cauliflower-like balls, usually two to six inches across, though specimens up to 30 inches are known, with a hard outer layer. Crack one with a sharp blow of a bricklayer's or rock hammer and the inside may be hollow and lined with crystals pointing inward, or filled nearly solid with crystal growth. Quartz dominates most of them, sometimes pink and grey; calcite is common too, in many habits and colours; and another 17 minerals have been identified in Keokuk geodes.

Most come from the lower Warsaw Formation, a widespread rock unit of Mississippian age, the Warsaw Shale that outcrops along stream beds in Iowa's southeastern counties, especially Lee, Henry and Van Buren. Fresh geodes can be dug from exposures where they are concentrated in certain layers, and where streams have cut into those layers the geodes weather out and gather in the channels.

The obvious place to go is the wrong one for digging. Geode State Park, 1,640 acres in Henry County on the Skunk River, joined the Iowa park system in 1937 and takes its name from the state rock, but few geodes can be found there, and removing them from any state park is illegal. A display of geodes in the park office shows what the creeks hold. The park's real draw is Lake Geode, built in 1950 and dedicated in 1951 with its beach and a beach house in the style of the Civilian Conservation Corps, whose crews had first cleared the ground and built its roads and structures. It is good water for largemouth bass, bluegill, crappie and catfish.

For the geodes themselves, the Iowa Geological Survey points out that most collecting grounds are on private land, so permission comes first. The Keokuk Area Convention & Tourism Bureau keeps a list of property owners who may let visitors dig, and with a little patience a creek bed can turn up a rock nobody has looked inside before.