What could this be? A piece of jewelry thats really old with black stone or gem?

[CrownVic26]

Hi everyone thanks for the read. I found this piece on a park near downtown Minneapolis that's surrounded by residential apartments. back in the 1840's this park was farmland before it was turned into a park in 1886. I had found what appeared to be a Clydesdale horseshoe from the same era when the farm used to be up and running. I also found this piece. I'm wondering what this could be. its the size of a modern quarter just a tad bit skinnier because of its oval shape. there is a swirl type design around what appears to be a gem maybe? although it is scratched on the top of the black piece all of the side edges has multiple faces (looks almost like the shape of a diamond with all the cut marks on the outside) it has little notches on four different places where it could have been attached to something somehow. and it weighs exactly 5 grams. Im curious if anyone knows any additional information about this? I only know the information provided which is strictly from the visible. Also wondering what this was used for and possibly an approximate age and value? it has historic value to me personally and would love to learn more about it. thank you for your time!

 

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[duggap]

With the color of the stone, it makes me think of mourning jewelry.