Sterling Silver with Marcasites and Onyx? Approx. Size 6

Sterling silver ring with Marcasites and what looks to me to be Onyx, marked 925. I don't know the makeup of the black stone, so please check out the pictures. Approx. Size 6; I show a pic of it on a ring sizer, so please check that out for precise size! Closet clean-out week! I'm selling a lot of jewelry and clothes, so please check out my closet. Bundle up for discounts, make me an offer! Thanks for looking!