The Prisoner in the Opal — A.E.W. Mason

An Englishwoman is brutally murdered in the South of France and an American goes missing. With the discovery of a body, a severed hand and an opal bracelet, Inspector Hanaud of the Surete is called in to investigate. It s not long before allegations of devil worship begin to arise. Inspector Hanaud must race against time before a second violent murder is committed."