Crater Of Diamonds State Park

The Crater of Diamonds State Park is an 888 acre (3.6 km2) Arkansas State Park situated over an eroded volcanic pipe. The park is open to the public and, for a small fee, rockhounds and tourists can dig for diamonds and other gemstones. Park visitors find more than 600 diamonds each year of all colors and grades. Over 24,000 diamonds have been found in the crater since it became a state park. Visitors may keep any gemstone they find regardless of its value (and some, as listed below, have been quite valuable).

More info at  http://en.wikipedia.org/wiki/Crater_of_Diamonds