Franklin W. Dixon 
Curse of the Ancient Emerald 

Frank and Joe must take the shine off a jewel thief who sends ominous riddles in this Hardy Boys adventure.First a priceless painting disappears from the museum; then two ancient samurai swords vanish from a private collection. A string of robberies has hit Bayport, and all signs point to the Phantom, a recently released ex-con who last struck more than twenty years ago. Theres only one problem: the Phantom has vanished into thin air, and not even Frank and Joe Hardy can track him down.Then the boys begin receiving threatening notes in the mail, each one containing a strange riddle. And it looks like the Phantoms next target could be the valuable Emerald of Astara, the 2, 500-year-old crown jewel of the Bayport Historical Museum. Can the Hardys crack the Phantoms code before he strikes again?