The Moonstone by Wilkie Collins

Beautiful young Rachel Verinder is presented with a large, rare Indian diamond on her eighteenth birthday, which was left to her in her uncle’s will. Disaster strikes when it disappears during the night of her birthday party, causing Rachel to become severely depressed. Her cousin, Franklin Blake, who was hoping to marry her, is devastated that she won’t even speak to him – and determined to solve the mystery of the stolen diamond.