Pearl Cove by Elizabeth Lowell

Pearl Cove (Donovan, Book 3) - When Len McGarry is murdered in his pearl shed, his widow Hannah calls on Archer Donovan to help her.  Several rival pearl farmers and merchants want the secret of the Rainbow Black pearls that Len was rumored to have produced.  Archer uses his family resources as well as his contacts with the US government to protect Hannah, search for her husband's murderer and find the Black Trinity necklace that Len believed would cure him.