Medically reviewed by Susan Bard, MD Melanonychia, also known as a dark or black line on the nail, causes a streak of dark ...

Common causes of Beau lines include infection of the nail bed or injury from impacts or picking. Contact your doctor if you’re sure a black line on your nail wasn’t due to trauma or it occurs ...

In mild cases, the black toenail will grow out naturally over time without treatment. In severe cases, such as when the nail begins to detach from the nail bed, a person should seek medical treatment.

And they can be quite painful. Subungual melanomas usually look like brown-black discolorations in the nail bed. The discoloration may be in the form of a long, thin line, or streak, or it can be ...