Carbonatite: Magnet Cove, north road bank just west of the Cove Creek bridge on AR-51; http://www.rockhoundingar.com/locations.php. The unusual igneous rocks in the Magnet Cove area are part of the Late Cretaceous (100-million-year-old) ring dike complex, the core of which includes this carbonatite, a rare igneous rock composed predominantly of calcite.
Crater of Diamonds State Park: 209 State Park Road, Murfreesboro, 71958; http://www.craterofdiamondsstatepark.com/. Diamonds are weathering out of an unusual rock called a lamproite, part of the Prairie Creek diatreme that rose rapidly upward through Earth's mantle and crust about 100 million years ago.
Crowley's Ridge State Park: 2092 AR-168 North, Paragould, 72450; http://www.arkansasstateparks.com/crowleysridge/. This park is located on Crowley's Ridge, a narrow ridge composed of windblown loess that extends from southeastern Missouri through much of northeastern Arkansas and rises in stark contrast to the otherwise flatlands of the eastern Mississippi Embayment.
Devil's Den State Park: 1333 W. AR-74, West Fork, 72774; http://www.arkansasstateparks.com/devilsden/. Located in a valley in the Boston Mountains, the caves such as the Devil's Ice Box did not form from dissolution of limestone but are instead the result of a rockfall in Paleozoic sandstone.