To do it:
Place several BB’s between two pieces of aluminum tape. Lay a penny over the tape and cover it with another piece of tape.
Hammer the BB’s into the penny. Remove the penny from the layers of tape.
Place the penny on a piece of wood and use the head of a carriage bolt to hammer in a convex shape.
Solder a nail to the back side of the penny and mount it in a drill press. Spin in round and use progressively finer sand paper to shape and buff it. Finish it off with polishing compound and a cloth.
De-solder the nail from the penny before you drill a hole in one of the divots.
Make a jump ring for it to place it on a chain.