That has been asked before:
If you have an original and not a reproduction you have a 1787 doubloon. I think there may be only a very few known samples of this coin. Value? Are you kidding? It has been written about in novels and had a movie made about it.
---} Click here for more info {---
---} Click here for more info {---
The Gallery Mint Museum sold reproductions of the Brasher Doubloon. A reproduction is probably what you have.