thanks for the replies....
i believe it is an old coin...no identifying features are left, and i have used a 10x numerous times to study the piece.
it weighs 15.7 grams, less than most coins, just shy of an 1870's peso.
the irregularities around the rim suggest to me it was made from something else.
the amount of work that went into this piece, hammer marks, also suggest it was not a flat sheet formed into a disk.
i have made conchos from flat sheet, and it is fairly simple, not requiring the amount of time spent on this piece.
i guess i am basing my opinion on the "feel" of the piece, the location of the find, and my own familiarity with navajo jewelry...
again, thanks for the response