The Jewel of the Nile

This is the sequel to "Romancing the Stone" where Jack and Joan have their yacht and easy life, but are gradually getting bored with each other and this way of life. Joan accepts an invitation to go to some middle eastern country as a guest of the sheik, but there she is abducted and finds her- self involved with the "jewel". Jack decides to rescue her with his new partner Ralph. They all go from one adventure to another... What is the story of this "jewel"?