Big white stone on ring?

Detectingman headed out to this park this morning in search of goodies. About 5 minutes in hit this half ring, which showed up as a solid 55. Anyone happen to know what type of stone this is? I searched for the band but it was not in the hole, must of been hit by a lawnmower. About a half of hour later hit on something, a solid 77. Ended up being a pair of brass knuckles. Never know what you will find dirt fishing