Another interesting find Crispin.
I once pulled in a snarl of fishing line with a cheap turquoise ring on it for a weight.
Various hardware has be found attached to other lines and more than one spark plug has gone fishing.
Hard telling what your find was used for. If for fishing a piece of mono leader would help. Or were cordage not so sash weight looking and hinted of being more old, a horse hair leader to hook. But then ocean fish may not all be hook shy...
First guess was a miner high grading and making his/her own bling but coin would be easier and few examples of raw silver in adornment are found in a short look.
Silver coin of little monetary value to the user could certainly have been re-purposed, or as G.I.B. suggests been a form of barter.