The Diamond as Big as the Ritz by F. Scott Fitzgerald

On a summer break from his New England education, John T. Unger of Hades, accompanies a new friend to his father’s private estate—a hidden palace carved out of one solid diamond mountain where extravagance knows no bounds, and everything feels a spectacular dream to modest Unger until he discovers the frightful lengths to which the family will go to protect their secret and their treasure.