Black Pearl, The (Vintage Ex-Lib Hardcover) by Scott O'Dell

Ramon cannot believe what he has just found in an oyster he's brought up from an underwater cave where the Manta Diablo, the monster devilfish, lurks. Ramon is holding a pearl. Not just any pearl, but the most fabulous gem he or anyone else has ever seen. But neither sixteen-year-old Ramon nor his father foresees the trouble that such a pearl can bring. It will be young Ramon who must stop the monster he has unleashed.