Melanonychia is a medical condition in which the nail plate has a pigmentation band that is either black or brown in color.
There are two main types of melanonychia, meaning longitudinal (this being often times a sign of subungual melanoma) and transverse.
This medical condition appears within other medical conditions that are benign, such as the common mole. The band of pigment appearing on a nail has to be properly investigated, as it might be a sign of melanoma.
It seems that individuals with darker skin and Japanese people present a higher risk, the percentage being reduced in the white population. As for age and gender, there is no particular category affected by melanonychia.