A dark streak in a nail may come from bruising, extra pigment, medicine, or, less often, melanoma under the nail.
A black line on a fingernail can show up after a small injury, from extra pigment in the nail, or from a nail disorder. Most dark streaks are not cancer. A new line on one nail, a widening band, or color that spreads onto the nearby skin should still be checked soon.
Why Do I Have a Black Line on My Fingernail? The Main Causes
Doctors often call a lengthwise dark band longitudinal melanonychia. That label describes the pattern. It does not name the cause. The cause can range from normal pigment to old blood to melanoma under the nail.
A pigment band from melanin
The nail grows from tissue under the cuticle called the matrix. When that tissue makes extra melanin, the nail can grow out with a brown, gray, or black band. In people with darker skin, this may be a normal finding. Some medicines, pregnancy, nail trauma, and a few health conditions can also trigger it. The MSD Manual’s nail discoloration page notes that these lines can be benign and can appear from the base of the nail to the tip.
Blood trapped after an injury
A slammed finger, sports hit, or tool slip can leave blood under the nail. That is called a subungual hematoma. It often starts with throbbing pain, then a dark patch or band appears. MedlinePlus nail injury advice says bruising under the nail can follow a crush or cut injury, and larger bruises may need treatment to drain trapped blood and ease pain.
A warning sign that needs a skin check
A dark line can also be melanoma in the nail unit. That is rare, but a new band on one nail needs care when it is getting wider, has uneven color, changes the skin next to the nail, or comes with nail splitting, lifting, or a bump under the nail. The American Academy of Dermatology’s nail melanoma signs list those same changes.
What The Pattern Can Tell You
The line cannot diagnose itself, yet its pattern can point you in the right direction. Check the width, color, number of nails involved, any pain, and whether the nearby skin changed too.
- One nail or many: one new band can be more worrisome than several steady bands.
- Pain or no pain: pain leans toward injury; many pigment bands do not hurt.
- Narrow or widening: a band that spreads needs faster care.
- Skin next to the nail: color on the cuticle or side skin is a red flag.
When To Book A Medical Visit
Some dark streaks can wait for a routine appointment. Others should jump the line. The split comes down to change, pain, and whether the skin near the nail is involved.
- The band is new and only on one nail.
- The line is getting wider or darker.
- The color reaches the skin beside the nail.
- The nail is splitting, lifting, or growing with a groove.
- You feel a bump under the nail.
- You do not recall an injury.
Get same-day care if you crushed the finger, the pain is pounding, the nail is partly torn off, the fingertip looks bent, or the bruise takes up a large part of the nail. MedlinePlus notes that bigger nail bruises and nail bed injuries may need drainage, repair, or an X-ray if there could be a fracture.
| Pattern you see | What it may point to | What to do next |
|---|---|---|
| Thin, even line on more than one nail | Benign pigment band | Bring it up at a routine visit |
| Dark patch or band after a recent crush injury | Blood under the nail | Same-day care if pain is strong or the bruise is large |
| Single dark band that is getting wider | Pigment disorder or melanoma | Book a skin or nail check soon |
| Uneven color in one band | Needs closer review | Do not wait for it to grow out |
| Color on the cuticle or side skin | Higher concern for melanoma | Arrange a prompt dermatologist visit |
| Nail lifting, splitting, or a lump | Nail unit disease | Get it checked soon |
| Band that has looked the same for a long time | Often a stable pigment change | Take photos and watch for change |
How A Clinician Checks A Dark Nail Band
A nail visit starts with the story. When did the line appear? Did it come after trauma? Is it on one nail or several? Has it widened? What medicines do you take? Those details can narrow the field fast.
At The Visit
The nail, nearby skin, and your other nails are usually checked side by side. A clinician may use magnification to study the band and its borders. Phone photos can help too, since nail change can creep up over time.
If There Was A Recent Crush Injury
If the story fits a bruise, the visit may center on pain relief, drainage, and whether the nail bed or fingertip bone was injured.
If There Was No Clear Injury
If the band is new, growing, uneven, or tied to darker skin near the nail, the visit may shift toward ruling out melanoma. In some cases, a dermatologist may sample the nail matrix with a biopsy.
| Your situation | Best next step | How fast |
|---|---|---|
| New line, no injury, one nail | Book a dermatologist or primary care visit | Within days to a couple of weeks |
| Dark bruise after a slammed finger | Ice it, rest the hand higher, and watch pain | Same day if pain is strong |
| Band widening or darkening | Arrange a skin check | Soon |
| Color spreading onto nearby skin | See a dermatologist | Promptly |
| Nail split, lift, or lump | Get it checked | Soon |
| Several steady bands on several nails | Bring it up at a routine visit | Routine |
What You Can Do Today
You do not need to sit and guess. A few plain steps can make the next visit more useful.
- Take a clear photo in daylight, then repeat it every few weeks from the same angle.
- Write down when you first noticed the line and whether you had a recent finger injury.
- Remove dark polish or gel so the nail and nearby skin are easy to inspect.
- Do not drill, file, or pick at the nail to see what is under it.
- Bring a medicine list, since some drugs can trigger pigment bands.
Mistakes That Can Waste Time
The biggest mistake is assuming every black line is a bruise. That guess is right often enough to fool people, yet not often enough to rely on. Another miss is waiting for a line to vanish when there was never a clear injury in the first place.
- Keeping the nail under polish for months and never checking it.
- Judging the line from one quick glance instead of tracking it with photos.
- Ignoring skin darkening near the cuticle or nail fold.
- Putting off a visit because the line does not hurt.
What This Usually Means
A black line on a fingernail is often benign. Pigment bands can be normal. Bruises under the nail are common after minor trauma. Those causes are far more common than melanoma.
Still, a new dark band on one nail, a line that grows wider, or color that reaches the skin next to the nail should be checked. When the story is not clear, an exam is the smart move.
References & Sources
- MSD Manual.“Deformities, Dystrophies, and Discoloration of the Nails.”Explains that melanin can form dark nail bands and lists benign causes and warning signs.
- MedlinePlus.“Nail Injuries.”Shows how bruising under a nail happens after injury and when a larger bruise may need treatment.
- American Academy of Dermatology.“How to Check Your Nails for Melanoma.”Lists warning signs such as a new dark streak, skin darkening near the nail, nail splitting, lifting, or a lump.