1. Divination with pearls, usually by either casting pearls or studying pearls in oysters: One ancient use of margaritomancy involved throwing a pearl into a cast iron pot sitting in a fire and watching it to determine a person's guilt or innocence in a crime. If the pearl started to move, then the person was believed to be guilty. If it stayed in place, then he or she was not guilty.
2. Etymology: from the Latin margarita, "pearl".