It's probably harmless, but it can be a sign of melanoma of the nail.
Associate Professor Anne Howard, dermatologist with the Australasian College of Dermatologists (ACD) who specialises in nails says: "Multiple lines are fairly common in dark skinned people. Bleeding under the nail can sometimes look very black- usually this is in the toenails, especially the large one. However a new dark line in a light skinned person's nail, especially the big toenail, thumb and index finger may be the sign of early melanoma of the nail."
Other signs of melanoma of the nail include:
Associate Professor A Johannes S. Kern, dermatologist with the ACD, who also specialises in nails, says: "Sometimes patients are not referred to the right specialist. Dermatologists specialise in the area of nails- not just skin- and are the best doctors to find the right spot to biopsy and diagnose if it is melanoma or not."
As with any cancer, staying educated and giving yourself regular self-examinations is crucial. Anything suspicious should be discussed with your General Practitioner (GP) as soon as possible, as the key to surviving melanoma is early detection. Your GP may refer you to a dermatologist if necessary.
A/Prof Howard says: "Treatments of this cancer vary on the stage in which it has been caught. If discovered early enough, the nail tissue or nail may only have to be removed. If the cancer is fairly well-developed, the entire tip of the finger may need to be amputated."
"One of my patients, a 52 year-old woman who developed a line in her nail which gradually widened, was busy working on a farm, but saw an article in a magazine about lines in nails so saw her GP and was referred to a dermatologist. She has a biopsy and it was an early melanoma. She had to have her nail and the skin around the tip of the thumb removed, but as it was an early melanoma she didn't have to have an amputation."