Vintage Sterling Marcasite & Onyx Pin (Broach)

Lustrous large rectangular onyx stone flanked by sterling fretwork inlaid with shiny marcasite stones. I can not find a 925 sterling stamp, but I feel fairly certain this is sterling. It was my mother’s, and I was with her when she purchased it many moons ago. Size shown in pics, and layed across my fingers for a size reference. Will arrive in a jewelry pouch attractively wrapped. Non smoking home