Bugs are wiping their asses on them
Without knowing more details could be many causes. Is this a chronic condition? Could be Raynaud's phenomenonhttp://www.answers.com/raynauds or combination of other related circulatory problems such as arthritis, polymyositis, carpal tunnel, drugs, exposure to cold, lupus etc. etc.
a smoker's teeth are yellow. - skin is yellow-ish - finger tips turn yellow. - if their breaths stink..
Starch.
Rusting
capillaries run into your fingers some end there but some turn around and go back down
it is not gold, it could be gold plated or brass
Mold
Yes, It can turn the crystal black and permanently damage it.
Gravitation.
Brake dust!
It mite be hereditary i have the same problem but on my finger go to the doctors they will help but its not a big of a deal.