Faraone Mennella—run by the design duo Roberto Faraone Mennella and Amedeo Scognamiglio—opens its newest jewelry boutique this month in Porto Cervo, Sardinia, where it will showcase a collection inspired by the style and ambience of Capri. The 18-karat-gold La Sirena ring, for instance, features the tail of a siren—a mythological creature that is half fish, half woman, and is said to inhabit the island of Capri—curled around the finger and carpeted in white diamonds ($6,960); while the Grotta Azzura ring—an 18…