What are summon stones?
Summon stones are stones filled with magic energy with which you can call and dismiss pets. The magic of a summon stone, however, only lasts for a certain number of attempts. Once the magic is completely spent, the summon stone will turn into a regular useless stone. The same is true if you try to summon a pet which you do not own. Check your "Quest Log" for more information on how to fill your summon stone with magic again.
Further note that summon stones can only be used once every 30 seconds.