A black line running lengthwise on your nail is usually a band of pigment produced by melanocytes, the pigment-making cells that sit in your nail’s growth center (the matrix). Most of the time it’s harmless, but in rare cases it can signal melanoma, so it’s worth understanding what’s normal and what deserves a closer look.
What Creates a Pigmented Nail Band
Your nail matrix contains roughly 200 melanocytes per square millimeter, and most of them stay dormant your entire life. When something activates these cells, they start producing melanin and passing it into the nail cells forming above them. As the nail grows outward, that pigment travels with it, creating a visible stripe that runs from the base of the nail to the tip. Doctors call this longitudinal melanonychia.
The key distinction is whether those melanocytes are simply more active (producing extra pigment while staying the same in number) or actually multiplying. Extra activity is almost always benign and can be triggered by everyday causes. An increase in melanocyte numbers can mean a mole in the nail matrix, which is also usually harmless, but it can occasionally indicate melanoma.
Common Harmless Causes
Skin tone is the single biggest factor. Pigmented nail bands appear in up to 77% of Black adults over age 20, about 20% of Japanese adults, and roughly 1.4% of white adults. If you have darker skin, a line on one or several nails is extremely common and typically nothing more than normal melanocyte activity.
Injury to the nail is another frequent cause. Stubbing a toe, slamming a finger in a door, or repetitive pressure from tight shoes can produce a subungual hematoma, which is blood trapped under the nail. These spots tend to look reddish-black or purple near the base and more streaky toward the tip. The key tell: a bruise moves forward as your nail grows out and eventually disappears. A true pigment band stays anchored in the same position at the base because the melanocytes producing it are fixed in the matrix.
Certain medications can also flip melanocytes on. Chemotherapy drugs, particularly those in the anthracycline and fluoropyrimidine classes, are well-known triggers. You might see a single dark band or widespread discoloration across multiple nails. The pigmentation often fades after stopping the medication, though it can take months for the nail to grow out completely.
Fungal and bacterial infections occasionally darken a nail, too. These infections tend to cause additional symptoms like nail splitting, thickening, or pain, which helps distinguish them from a simple pigment band.
When a Black Line Could Be Melanoma
Subungual melanoma is rare, but it’s the reason any new, solitary dark band on a nail deserves attention. Dermatologists use an ABCDEF system to evaluate suspicious lines:
- Age: Peak risk is between 50 and 70 years old. People of African, Asian, and Native American descent are disproportionately affected; nail melanoma accounts for up to one-third of all melanoma cases in these groups.
- Band appearance: A brown-to-black band 3 mm or wider with blurry or irregular borders raises concern.
- Change: A band that gets wider, darker, or more irregular over weeks to months is a red flag. Equally concerning is a nail that doesn’t improve despite treatment for a presumed infection or injury.
- Digit: The thumb, big toe, and index finger are the most commonly affected.
- Extension: Pigment that spreads beyond the nail onto the surrounding skin (called Hutchinson’s sign) is treated as a sign of malignancy unless proven otherwise.
- Family history: A personal or family history of melanoma or atypical moles increases risk.
No single criterion confirms melanoma on its own, but the more boxes that are checked, the more urgently a dermatologist will want to examine the nail.
How to Tell a Bruise From a Pigment Band
This is the most common point of confusion. A bruise under the nail typically appears after a specific injury you can remember, looks reddish-purple rather than brown-black, and migrates toward the tip of your nail over the following weeks. Fingernails grow roughly 3 to 4 millimeters per month, so a bruise near the base of a fingernail can take three to six months to fully grow out. Toenails grow about half as fast, so the process takes even longer.
A true pigment band, by contrast, doesn’t move. It stays rooted at the base because it’s being continuously produced by active melanocytes in the matrix. If you’re unsure, marking the position of the discoloration relative to the cuticle and checking again in a few weeks can help clarify whether it’s migrating forward or staying put. One important note: a bruise can sometimes hide an underlying melanoma, so if the spot hasn’t grown out or changed as expected after several months, it’s worth getting checked.
What Happens During Evaluation
A dermatologist will typically start with a dermoscope, a magnifying lens with a light that reveals pigment patterns invisible to the naked eye. Irregular patterns under dermoscopy, where pigment lines vary in color, thickness, or spacing, are the primary trigger for a biopsy.
Beyond the dermoscopy pattern, certain situations raise the threshold for biopsy on their own: a new solitary pigmented band that appears for the first time between your 40s and 60s, pigmentation that develops suddenly on a previously normal nail, a band that darkens or widens over time, or any pigment extending onto the skin around the nail. Periungual pigmentation (discoloration of the nail fold skin) is considered a sign of malignancy until a biopsy says otherwise.
If a biopsy is needed, it’s taken from the nail matrix itself. This is a minor procedure done under local anesthesia, and because the matrix is where nail cells are made, there’s a small chance of a permanent ridge or slight deformity in the nail afterward. Still, the diagnostic information gained outweighs the cosmetic trade-off when melanoma is a possibility.
What You Should Watch For
If you have dark skin and notice faint brown lines on multiple nails, that’s almost certainly ethnic melanonychia and perfectly normal. A single dark band on one nail, especially if it’s new, is the pattern that warrants closer attention.
Take a photo of the nail next to a ruler and check it again in four to six weeks. Widening, darkening, or pigment spreading onto the surrounding skin are the changes that should prompt a dermatology visit. The same goes for any band that appeared suddenly on a previously clear nail, particularly if you’re over 40 and have no history of injury to that finger or toe.