Found this today along a low river bank.. appears to be silver with a place where a pin went on the back so I'm guessing it's a vintage silver brooch?but not sure.. see pics
Judging by just the 2 photos, I think your piece is Marcasite jewelry. Marcasite jewelry is made from Pyrite, which is not as brittle as Marcasite.
If you post more photos of the piece, I can better judge what piece of jewelry it is. You might want to take the photos while it is lying down (not holding it.)