An example of a 999 silver coin is the Guernsey Eight Double one-ounce fine silver coin.
Per Wikipedia, The Bailiwick of Guernsey (French: Bailliage de Guernesey) is a British crown dependency in the English Channel off the coast of Normandy. It has a warm almost temperate climate as is warmed by the gulf stream Guernsey has it’s own mint and produces some very nice gold and silver coins.
Whereas most countries feature the current head of state, the State of Guernsey uses a Guernesiais motif, hence the ancient seal of King Edward I given to the island in 1279 that featured on the Doubles. The original seal bore the three lions of England, crowned later to become leopards, surmounted by a branch to represent broom or 'planta genista', Edward's badge of rank. Above the seal was 'Guernesey' in French, the legal and everyday language of the island at that time.
The Eight Doubles is legal tender and first minted in 1830; it was released periodically until 1966 together with the One, Two and Four Doubles.
The term 999 comes from the fact that the coin is 99.9 percent pure silver (or sometimes gold).
A 999 Guernsey Eight Double fine silver coin usually costs around 12 British pounds or around 20 US dollars.
One troy ounce
Purity 999 fine silver –
Weight - 31.1035 grams
Coin Edge - Plain
Although these coins were minted for a number of years, the quantity available is limited and they would therefore make a fine addition for a serious collector and the Guernsey Eight Double is a fine example of a 999 silver coin.