Coins lose virtually ALL value when the original patina is destroyed. Toning is a normal process found on coins. Those coins cleaned  with any type of commercial cleaner--i.e. cleansers, copper cleaners, silver polish, dips, etc., completely lose the original patina along with most discloration, spots, etc.---but not permanently. When the coin starts to retone, there will be no vestage of the original surface patina but usually a visual panorama of strange colors and shadings and the realization that the coin's been cleaned! AND----the coin's value has been diminished or disappeared.     If a coin or coin-related item is really dirty, give it a soaking in some warm water with a touch of detergent in it.  Rinse thoroughly and pat it dry. Handle the coin by holding it at the edges at all times as the oil in one skin transferred to any coin surfaces caused surface damage to coins.  REMEMBER--never use any commercial cleaners on any coin surfaces. Polish your car; the furniture; the chrome on your car--but never your coins, medals, tokens, or any exonumia!