פירוש על עבודה זרה 94:17

Ami was not referring to the forming of the stone but to its annulment. What he said was that even if the person had put the plaster or tile in the stone, if he removes it the stone goes back to being permitted (it is annulled). We might have thought that since the plaster was in the stone, it was like making the stone for idolatrous purposes, so R. Ami teaches us that it is not.