Hello. I Have A Question About Dark Bands Associated With

Hello. I have a question about dark bands associated with fingernails. I understand that a dark longitudinal band on the nail bed is something that should be checked by a doctor as soon as possible. However, I'm wondering about dark bands on the nail itself. I have a slightly dark, around 1-2mm thickness, running longitudinally on the nail itself of my index finger, which has been there for over 2 months now with no chance. Do you have any opinions on this? Thank you.