The Diamond as Big as the Ritz is a novella written by F. Scott Fitzgerald, ultimately included in his compilation Tales of the Jazz Age.
The story tells of John T. Unger, from small-town Mississippi, who while at boarding school becomes the only friend of the reclusive Percy Washington. Unger is invited by Percy to his home "in the West", while hearing Percy brag that his father owns a diamond which is bigger than the Ritz-Carlton Hotel.
Remarkably, Percy's boast is true: Percy's grandfather discovered a mountain comprised of one solid diamond. Due to the dilemma his grandfather faced (the diamond alone based on current prices would be worth trillions, but the law of supply would result in diamonds being worthless), the family goes to appalling lengths to keep the secret (the business is run by descendants of blacks who think the South won the Civil War, and are therefore still slaves; airmen who stray into the area are captured and held prisoner, and visitors are killed with their families told they died of illness). Unger learns from Percy's sister Kismine (with whom he has fallen in love) that he too will be killed; that night the house is bombarded by airplanes courtesy of an escapee. Percy's father offers to bribe God; failing that he blows up the mountain (along with himself, his wife, and Percy), while Unger, Kismine, and Jasmine (Percy's other sister) escape, though now penniless.