Crater of Diamonds State Park in Arkansas is fairly small as state parks go, but it is also unique among state parks. It is a gem mine open to the public for a small fee. Originally a commercial mine site, it was the source of the largest diamond found in North America, but was not very profitable, and it was sold to the government of the state of Arkansas in the 1970's or 80's and turned into a state park. For a small fee the gem field is open for searching and digging by members of the general public, and it produces not just its namesake diamonds but a wealth (pardon the pun) of other gem stones. A teenager just discovered a 7.44 carat diamond there. The local news in the area covered the story. Neat.