In it's purest form, silver is indicated as .999, but silver alone is actually too soft to be used for jewelry making without mixing it with other metals. When metals are combined, they are referenced as alloys. Jewelry marked .925 Sterling Silver, is an alloy of silver containing 92.5% pure silver and 7.5% other metals, usually cooper, to make the metal strong enough for crafting fine jewelry. This ratio is called the "sterling standard" and has been used in the US since the mid-1860s. Silver tarnishes when it is exposed to air as sulfur compounds in the atmosphere, principally hydrogen sulfide, chemically react with the silver alloy. Normal friction of frequent wearing reduces the development of tarnish, so silver jewelry that is worn regularly will not require polishing very often. When not in use, items crafted of the purest alloys of silver benefit from being stored in silver cloth in a dry place. Silver cloth is a a flannel-like fabric impregnated with microscopic particles of silver or zinc. The metal particles in the cloth attract and absorb sulfur compounds before they can reach the silver item being stored. When you want a piece of jewelry with brilliant shine, this simple proper care of polished sterling silver is well worthwhile.
"Oxidized Sterling Silver" has been exposed to sulfur in a controlled manner to impart a patina with a rich, organic character and depth similar to the patina of vintage silver. It differs from silver that has not been oxidized in that it is a darker color, is not as shiny, and is not affected by tarnish. Oxidizing makes inscriptions and patterns stand out nicely. Oxidized silver does not have to be polished or cleaned and does not have any special storage requirements. The only care oxidized silver needs is occasional cleaning with soapy water and gentle drying with a soft cloth (not silver cloth which will remove the oxidized finish).