My father collects coins as a hobby and lately has observed some green spots in some of his coins so he told me to ask here to see if anyone knows what it is.
The coins are made of silver (999 parts out of 1000 according to the description) so at first he thought that it might be some silver oxide but the silver oxide is dark brown and not green.
He looked it up in the Internet and he has read that it might be copper chloride but it doesn't seem right to him due to the purity of coin.
Is this feasible? Can the spots be copper chloride? If not, what can they be?