To me it looks to be made of brass or at least one of the bronzes. It is oxidised with age, but when new it would have shone brightly. The cone shapes on the front would catch the light and make it sparkle, but you are right they could be used as settings for stones - but not diamonds or it would be silver at least. Most likely garnets or rock crystal. The back is typical for small cast pieces - and you may find fingerprints because it was cast from a wax form. (Lost wax method).
And I think it may be a shoe buckle. Not for holding the shoe on, as there were laces for that, but for decoration. This would explain the small clip, but would also be though a slot in the leather to hold it in. The problem now is that heavy ones like that are a couple of hundred years old, So the mystery of how it got there becomes more of a problem.
Possible it is younger, but not much than the 1920's. Things got lighter and more decorative after that, but it could go back to the 1800's.
Could be a show and tell item that was lost in the ether.