Surprisingly, most keys aren't merely stamped pieces of metal. Typically, keys that you normally see today are made of a mixture of either nickel, brass, silver, or a bit of metal. Most often, keys are an alloy between at least two of these materials, usually a brass-nickel alloy. The pins on the locks that those keys go into are also usually made of brass. Given that brass is a soft metal, it's no wonder why keys and pins can get worn down.