The stone he put in its place looked exactly like Jon's stone, except that it wasn't. It was the Gothic Stone. Instead of granting wishes normally, like the regular wishing stone, this stone twisted the results of the wishes made, so that they became gothic.
For example, if Karyn simply wished for a dress, the result of the wish might be a black tattered witch-like dress. Or if Jon wished for a new car, the result of the wish might be a hearse.
Other than the twisting of the results, the stone had all the same rules as the regular wishing stone.