South Yorkshire Police say that two girls, aged between 10 and 16, took the ring from the swimming pool changing rooms.
It happened on May 13 at around 7.20pm. The first was described as white, of a stocky build and approximately 4ft 8ins tall, with red hair. The second was also white, of a medium build, around 5ft 3 ins tall with brown hair.
Anyone with information should call 101 quoting investigation number 14/89305/22 of 13 May.