Being bright and shiny is not the natural state for most of the metals your coins are made up of. The formation of a patina is the natural consequence when a piece of refined metal (such as a coin) is kept in an oxygenated environment (like the open air) for a prolonged period.
Gold Certificates asked:
Is patina a bad thing?
That depends on what kind of coin we're talking about.
On modern proof and uncirculated coins, patina shouldn't have had time to form yet, especially if the coins have been stored properly. On such coins, "patina" would be regarded as a sign of poor storage, environmental damage (such as water or fire) or an attempt at creating artificial toning.
On older coins and coins from circulation, patina is considered normal and natural, and it's removal (by "cleaning") is generally frowned upon by coin collectors.
On ancient coins, patina is inevitable, and depending on the composition of the coin, the patina can be quite thick. All ancient coins need to be cleaned after they're dug out of the ground, but an ancient coin which has had it's patina completely stripped away to reveal the bare metal underneath is considered to have been overcleaned, and not as desirable as a coin with it's patina intact.
Gold Certificates also asked:
Also can you get Patina on gold? If so, how do you get it?
Gold is one of the few metals which is naturally resistant to oxidation at normal atmospheric temperature and pressure. Indeed, it is quite resistant to most kinds of chemical attack. As a result, pure gold doesn't form a patina. Slightly diluted gold, such as the .900 fine alloy used for American circulating gold coins, can discolour slightly because the copper in the alloy can form a patina. Gold so impure that a "real patina" readily forms has rarely been used as a coinage metal.
It probably is possible to create an "artificial patina" on a pure gold coin, but you'd need (a) some pretty noxious chemicals, (b) some high-tech gear, and/or (c) a lot of very expensive trial and error.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis