What is a black vertical grain on fingernails?

It is normal for fingernails to show vertical lines, but if they show pigment, they may be melanotic moles. If they grow in the position of the nail mother, they will show black vertical lines when they grow outward. This phenomenon may be the early stage of melanoma, so be sure to go to the hospital for further examination. But if it is only inconsistent with the color of the deck, it may not be treated.