wow, just wow. sooo much wrong in this thread...
sterling silver is .925 silver remainder copper and trace elements. absolutely no platinum or rhodium. it was sometimes plated with rhodium, but not any more with the price of it now.
"fine" silver, as snowman says, is the trade term for .999 silver, usually electrolytically refined. fine silver does NOT oxidize much, to the point you can melt it, cool in air, and it stays bright and shiny, kind of freaky if you haven't ever done it. it IS highly reactive with sulfur, and "tarnish" on common silver items is a combination of copper oxide and silver sulfide. the copper in common alloys does react readily with O2, and also absorbs a LOT at molten temps. (I think a lot more than silver).