These labret studs have a 4mm flat disc backs and are internally threaded 316L surgical steel posts with a brass pvd and rhodium plated chevron arrow filled with round cubic zirconia gems.
Because brass contains copper, it can oxidize with moisture or skin chemistry and leave a harmless greenish tint on the skin. It isn’t an allergic reaction, but it does make these pieces better suited for occasional or event wear rather than daily use.