According to Longman Online Dictionary: "In British English, a stone is a piece of stone small enough to pick up. In American English, this can be called either a rock or a stone."
看解释。stone:a piece of stone shaped for a purpose, esp. one of commemoration, ceremony, or demarcation 石块 rock: a large piece of rock that has become detached from a cliff or mountain; a boulder 岩石
To get into a certain depth, we must be competent to know the concepts and knowledge and grasp what is said in a dictionary. Thus, sometimes an encyclopedia is helpful.