I've had the same problem in the UK. Our nickel-brass pound coins tone to a dark brown color when buried, and, if you try to spend an uncleaned one, people tend to assume it's a forgery: everyone knows there are lots of fake pound coins in circulation, but most people wouldn't recognise a fake one if they saw it. The cupro-nickel 5, 10, 20 and 50 pence coins also go dark brown. If I'm on the beach, I often rub them in wet sand between my thumb and forefinger: this usually gets them clean enough to spend. Sometimes however I just re-circulate them by paying them into the bank in bags with other coins, or by using them in vending machines. Coins that are really badly damaged, such as those cut in half by mowers, I usually keep as curiosities!



















