Subject: Why does gold leave a black mark on the skin of some people?

I am a student teacher at a high school in NC.  One day my kids
came in with an interesting question.  One student had taken their 
10K gold ring off and drug it across their hand and it left a black 
streak...We tried everyone in the classroom and could find no
pattern in the effects.  We tried a 14K ring and it worked also.  It 
seems to not have anything to do with skin color....and I think it's
hereditary because it works on myself as well as my mother.  The 
next day the kids came in with the same results.  Many of the
parents of the students affected were also effected....Please Help....I 
cant find the answer anywhere....Thank you

Re: Why does gold leave a black mark on the skin of some people?