The mandolin is a popular motif in jewelry, and this is a lovely example. It's probably a period imitation of the Mandel piece - this one is of course unsigned, manufacturer unknown. It's nicely decorated with clear rhinestones, and the mandolin body is enameled black. There is some loss of plating on the back - see photos - but the front is nicely preserved, with all rhinestones. 3.25" long, probably 1950s.
This brooch appears on page 123 of "Unsigned Beauties of Costume Jewelry" by Marcia "Sparkles" Brown.