Margaritomancy is divination with pearls, usually by either casting pearls or studying pearls in oysters. The word is derived from the Latin margarita, meaning pearl.
One ancient use of margaritomancy involved throwing a pearl into a cast iron pot that was sitting in a fire and watching it to determine a person's guilt or innocence in a crime. If the pearl started moving then the person was believed to be guilty. If it stayed in place they were not guilty.
References
Margaritomancy Wikipedia(Text) CC BY-SA