All by Myself, Alone - Mary Higgins Clark

Fleeing the disastrous and humiliating last-minute arrest of her fiance on the eve of their wedding, Celia, an expert on gems and jewelry, is hoping to escape from reality on a glamorous cruise ship.

But it is not to be. On board in the most luxurious suite is the elderly and world-famous Lady Emily Harworth. Immensely wealthy, Lady Em is the owner of a priceless emerald necklace that she intends to leave to the Smithsonian on her death.

Three days later Lady Em is found dead--and the necklace is missing. Is it the work of her apparently devoted secretary, or her lawyer-executor, both of whom she had invited on board for the cruise?

Celia, with the help of her new friends Willy and Alvirah Meehan--who are splurging on their wedding anniversary--sets out to find who the killer is, not realizing that she may have put a target on her back.