Eternity ring - Patricia Wentworth

Mary Stokes was walking through Dead Man's Copse one evening when she saw, in the beam of a torch, the corpse of a young woman dressed in a black coat, black gloves, no hat and an eternity ring set with diamonds in her ear. But when she and Detective Sergeant Frank Abbott went back to the wood the body had vanished.

This would have been mystery enough for Miss Silver to solve if a woman had not also reported that her lodger had gone out on Friday dressed in a black coat, black beret, black shoes and large hoop earrings 'set all round with little diamonds like those eternity rings.' She never came back...