Dark lines or streaks:
Black or dark vertical lines under the nail should never be ignored. While they can sometimes result from minor injuries, they may also indicate more serious conditions, including melanoma (a type of skin cancer). If the line changes in size, color, or shape, seek medical attention promptly.
White lines or spots:
These are often harmless and commonly caused by minor nail trauma. However, persistent or widespread white lines could signal low protein levels or zinc deficiency.