Nail Matrix Nevus Springer Nature Link This dark line or discoloration on the nail can be a common condition called melanonychia, basically, extra pigment in the nail plate. In many people, especially those with more melanin in their Dermoscopic features of nail matrix nevus (NMN) in adults and children: A comparative analysis ScienceDirect Melanonychia is brown or black discolouration of a nail. It may be diffuse or take the form of a longitudinal band. Melanonychia can present in individuals of all ages, including children, and