The Cutting Edge by Jeffery Deaver

DANGEROUSLY GOOD. DISTINCTIVELY DEAVER.
Lincoln Rhyme and Amelia Sachs return to New York City to confront a killer terrorizing couples at their happiest–and most vulnerable.

In the early hours of a quiet, weekend morning in Manhattan’s Diamond District, a brutal triple murder shocks the city. Lincoln Rhyme and Amelia Sachs quickly take the case.

Curiously, the killer has left behind a half-million dollars’ worth of gems at the murder scene, a jewelry store on 47th street. As more crimes follow, it becomes clear that the killer’s target is not gems, but engaged couples themselves.