Penannular 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, Global Gemology & Appraisals
The word penannular means "incomplete ring".

Penannular brooches, also known as Celtic brooches, are circular with a sliding pin used to secure a scarf or shawl.