Celtic Brooch

Antique, Victorian era penannular brooch (Celtic brooch) set with vivid orange citrines, pale yellow citrine, and jasper set in hand chased sterling silver.
Photo courtesy of Scott Papper, GIA GG, AJP, CM (NAJA), Global Gemology & Appraisals
Celtic brooches, also known as penannular brooches, are a type of clothing fastener.

The word penannular means "incomplete ring".