Margaritomancy is divination

Divination

Divination is the attempt to gain insight into a question or situation by way of an occultic standardized process or ritual...

 with pearls, usually by either casting pearls or studying pearls in oysters. The word is derived from the Latin

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 margarita, meaning pearl.

One ancient use of margaritomancy involved throwing a pearl into a cast iron

Cast iron

Cast iron is derived from pig iron, and while it usually refers to gray iron, it also identifies a large group of ferrous alloys which solidify with a eutectic. The color of a fractured surface can be used to identify an alloy. White cast iron is named after its white surface when fractured, due...

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.