Can I just start by saying I LOVE metal detecting! The best part is unearthing finds like this! I would love to see images of the piece in it's entirety if you could post some?
I can tell you that your piece is certainly 18k gold, the 750 backs the 18k mark by indicating that it is 75 % gold. If it were not gold, the minerals in the dirt would have certainly caused decay to the piece. It is likely that the diamonds could be authentic too.