Charles Forgelot silver ring,possibly.

Hi all,hoping someone out there might be able to throw some light on what this ring is and by whom,I’ve done some research and I’m leaning to it being by French silversmith Charles Forgelot but I’m not sure by all means,my problem is the hallmarks are very worn and unusually are stamped on the outer of the shank,I’m afraid my camera couldn’t take a good enough photo of the marks so I’ll have to describe them,firstly I think is the duty mark but I can’t make that out,secondly inside the diamond shape looks to be “C.Y.F” I can make out the C and the F but the middle bit might not be a Y and possibly a symbol ? I think the stone is probably a crysoprase along with the marcasite but again I’m guessing ? What’s your thoughts ?
Thanks