1930 S Buffalo Nickel Alloy

Soaked in a corrosive solution. The grain pattern is caused by the stretching of the metal during the rolling process especially of there is a slight non-homogenousness to the alloy. That would mean that some of the stretched areas are a slightly different alloy than the others and they corrode at a slightly different rate. It has been in the solution long enough to dissolve away enough metal to give you the reduced weight.