I purchased a Masada bronze coin in Israel for my son. We received a certificate of authenticity so we believe it is an authentic ancient coin. We asked if my son could wear it anywhere (as it was set in a pendant bezel) and the dealer said it is safe to wear in the shower as it has withstood thousands of years. Now, a year later, the coin has gotten blacker and is very hard to see the etchings. What can I do to remove the black so I can see the coin markings again?