Vintage Unique cameo tortoise brooch

Vintage cameo tortoise brooch. Older C clasp pin is bent but secure. I don’t think this is shell but it is unique with the tortoise coloring. Possibly lucite?? The only hallmark is the number 35 on the C pin clasp. The only mark I could research with a 35 was possibly a stamp "835" imprinted on silver is the purity mark used for silver items produced in the European countries. 657