What are the dark stripes that run down the nail bed of fingers and toes called,what deficiency causes them?

I didn't bump my finger or my toe, but I have dark lines that are only on certain fingers, my son gets the same thing, I'm wondering what vitamin deficiency produces these dark lines.What are the dark stripes that run down the nail bed of fingers and toes called,what deficiency causes them?
An onset of streaky, painless dark lines on your fingernails 鈥?not related to bruising or an injury- merits a trip to the doctor, as it could be a sign of melanoma.