This is the Avon "Modern Elegance" (also known as "Art Deco Revival") convertible pendant necklace, released circa 1980–1982.
It features a gold-tone fan or shell-shaped geometric design with black enamel (or acrylic) inlays and a central square-cut clear rhinestone. A key feature of this piece is often its convertibility— the black insert is removable to change the look.