Hopefully this has resolved itself now?
However I'm just letting you know that one of mine occasionally does this, a black moor, and (now I'm sure this will be looked on with horror by respected goldfish breeders, owners etc!) but I scoop him out with the fishnet for a few seconds, he gets so agitated (obviously!) and starts thrashing, which dislodges the stone.
There probably is a way to properly get stones out of greedy goldfish gobs, but in my panic in the times he does this it's very effective!!!!