Silver half crown from the HMS Association dated 1697 with silver rope twist chain.
The coin is held within a four clasp mount with the legends around it reading 'A TREASURE FROM ASSOCIATION 1707'. It is not soldered in.
HMS Association was a 90-gun second-rate ship of the line of the Royal Navy, launched at Portsmouth Dockyard in 1697. She served with distinction at the capture of Gibraltar, and was lost in 1707 by grounding on the Isles of Scilly in the infamous maritime disaster of the age. 
The coin is from the William 111 reign 1689-1702.