The story concerns a young woman called Rachel Verinder who inherits a large Indian diamond, the Moonstone, on her eighteenth birthday.
The book is widely regarded as the precursor of the modern mystery and suspense novels. T. S. Eliot called it 'the first, the longest, and the best of modern English detective novels'. It contains a number of ideas which became common tropes of the genre: a large number of suspects, red herrings, a crime being investigated by talented amateurs who happen to be present when it is committed, and two police officers who exemplify respectively the 'local bungler' and the skilled, professional, Scotland Yard detective. (Summary from Wikipedia)