S80 is a name many disingenuous merchants use to sell items that are either poor-quality silver-plate or not silver at all. While the name S80 implies an 80-percent silver content, items with this mark are either silver-plated or made of an entirely different metal.
Silver produced for sale in either the European Union or the United States requires several hallmarks that identify the product's origin and composition. The most important of these hallmarks is "925." 925 indicates that the item meets the requirements for sterling silver. 925 refers to the fact that 925 of every 1000 parts of the product, by weight, are pure silver. 925 is an officially recognized mark used by silversmiths worldwide.
S80 is not an official mark at all, and no legitimate silversmiths stamp their products with this hallmark. While items with the S80 mark frequently contain some silver, they are often plated with impure silver alloys. Aside from having very little inherent value, impure silver alloys frequently turn black and turn any skin they touch green.
Cheap silver-plate jewelry turns skin green because of a chemical reaction that happens between copper and oxygen in the presence of moisture. Copper is one of the metals used in making inexpensive silver alloys such as those found in S80 products.