This is a mystery novel. A very early mystery novel -- perhaps one of the proto-mystery novels. For one thing, it really does revolve about the title gemstone.
It's also an epistolary novel. We have a preface, a letter where a man explains why he is no longer willing to be on friendly terms with a cousin, which recounts the origin of the great diamond known as the Moonstone, leading into the actual events of the story. The first part is reported -- at length -- by an old retainer of the household, about the loss of the diamond. Or rather, starting with the explanation of the obviously ill-meant birthday gift of the diamond to the young heroine, and the mysterious advent of three Indian jugglers in the neighborhood, the ill-starred birthday dinner, the disappearance of the Moonstone overnight, and the todo over trying to find out what happened.
It veers into more points-of-view as we get the second half, the discovery of the truth. And we also get some great characters. The heroine, Rachel, and her mother, in particular.