St. Catherine of Alexandria, Caravaggio, Image taken from the Public Domain
Fillide Melandroni was a Roman prostitute Caravaggio often used to model for paintings of Mary Magdalen or popular Saints. His decision to always include the deformity on her left ring finger epitomized the striking realism that caused many supporters of an idealized Renaissance art form popular at the time to denounce him as a mere imitator of nature without merit. Those critics would have been completely ignorant to his use of blackness to shape and select details of the mise-en-scene (the scene more or less). It could be possible that Fillide sustained the injury as punishment for the time she tried to kill another prostitute who'd earned the favor of her pimp after a wild chase. Violent times.
Graham-Dixon, Andrew. (2010). Caravaggio: A Life Sacred and Profane. New York, W.W. Norton & Company, Inc.