for Silvestre y la Piedrecita Magica by William Steig
Sylvester the donkey finds a beautiful pebble with the power to grant him any wish as long as he is holding it. When confronted by a ferocious lion, Sylvester wishes to be turned into a rock and loses hold of the pebble as the wish is granted. He is now faced with the possibility of never seeing his parents again!
Publisher description retrieved from Google Books.