A Dark Streak Under a Nail — Bruise or Warning?

What a dark streak under a nail usually is

A dark line running from the base of a nail toward the tip is called melanonychia — pigment laid down inside the nail plate. Most of the time it has a harmless cause: a bruise from a knock you may not remember, a normal band of pigment that many people with brown or Black skin simply have, a reaction to a medication, or a benign mole in the nail's growth center. But a melanoma can also begin in the matrix, the living tissue at the base of the nail that produces it 1Ref 1National Cancer Institute (PDQ Adult Treatment Editorial Board) (2025).Melanoma Treatment (PDQ®)–Health Professional Version.Melanoma is a cancer of melanocytes that can arise in the nail matrix (subungual/acral melanoma); basis for statements on melanoma biology and its diagnosis and surgical treatment..

The most common look-alike is a bruise under the nail, a subungual hematoma. A bruise usually follows an injury, starts red-purple or black, and — this is the tell — grows out with the nail, drifting toward the fingertip over weeks until it clears. Pigment that stays anchored at the base, or a band that has sat in the same place for months without moving, behaves differently.

Several benign causes tend to affect more than one nail at once. Faint pigment bands across several nails are common and often reflect normal skin tone rather than disease; some medications can do the same, and repeated friction — a shoe rubbing a toenail — can darken a nail too. It is the single, new band in one nail, in an adult, that stands out from these patterns and earns a closer look.

The color of the band tells you less than people expect. Brown and black are typical of pigment, but a bruise can also look brown as it ages, and a perfectly benign band can be quite dark. What separates the harmless from the worrying is behavior over time and the pattern across nails — not the shade you see in a single glance.

The signs that separate a bruise from a warning

No single feature proves a nail streak is or isn't dangerous, but a handful of patterns raise concern together: a dark band in only one nail rather than several; a band that is getting wider, especially wider at the base than at the tip; blurred rather than sharp edges; more than one shade of brown or black; and any change over time. Change is one of the most reliable warning signs across every kind of melanoma 2Ref 2Tsao H, Olazagasti JM, Cordoro KM, et al. (2015).Early detection of melanoma: reviewing the ABCDEs.Change or evolution over time is a clinical warning feature supporting early detection of melanoma..

Read against a likely bruise, the differences line up like this:

• One nail, not many. A single new band draws more concern than faint bands across several nails, which more often reflect normal pigmentation.
• Width and widening. A band more than a few millimeters wide, or one visibly widening over weeks, matters more than a thin, stable line.
• Wider at the cuticle. A band that is broader where it emerges from the base than at the tip suggests a growing source in the matrix.
• Blurred edges and mixed color. Sharp, even edges are more reassuring than a smudged border carrying several shades.
• No injury, or a high-stakes nail. A band that appears with no remembered trauma, or on a thumb or big toe, deserves attention.

Of all of these, widening over time tends to carry the most weight, because a benign band is usually stable while a melanoma keeps growing. But the honest limit is that these features overlap, and none of them, alone or together, can confirm or rule out cancer from the outside.

A bruise grows out with the nail; a melanoma tends to stay anchored at the base.

The Hutchinson sign

One finding clinicians weigh heavily is the Hutchinson sign — brown or black pigment that spreads from the nail onto the surrounding skin, the cuticle, or the nail fold. When pigment crosses off the nail plate onto that skin, it raises concern that a melanoma in the nail unit is extending beyond it. Subungual melanoma is a form of acral melanoma, the type that arises on palms, soles, and nails rather than sun-exposed skin 1Ref 1National Cancer Institute (PDQ Adult Treatment Editorial Board) (2025).Melanoma Treatment (PDQ®)–Health Professional Version.Melanoma is a cancer of melanocytes that can arise in the nail matrix (subungual/acral melanoma); basis for statements on melanoma biology and its diagnosis and surgical treatment..

The sign is useful but not absolute. It is not present in every subungual melanoma, so its absence does not rule the diagnosis out — and a few benign conditions can push pigment onto the nail fold in a way that mimics it, sometimes called a pseudo-Hutchinson sign. That is why the finding is treated as a reason to be examined, not a diagnosis on its own: pigment spreading to the nail cuticle is a prompt to act, not a verdict.

A dark vertical line on a fingernail that also shows this spread is exactly the picture a dermatologist wants to evaluate in person. It is one of the features that separates a band worth watching from a band worth investigating, and it is easy to miss unless someone looks closely at where the pigment stops.

Why subungual melanoma is often found late

Melanoma under a nail is frequently mistaken for a bruise, a fungal infection, or a wart, and that delay is the real danger. Melanoma caught while it is still thin and confined to the skin is highly survivable; once it has grown deeper or spread to lymph nodes or beyond, survival falls sharply 3Ref 3National Cancer Institute, Surveillance, Epidemiology, and End Results (SEER) Program (2025).Cancer Stat Facts: Melanoma of the Skin.Melanoma survival is far higher when the cancer is detected early and localized than after it has spread regionally or to distant sites.. Because the nail hides the lesion and people wait for a 'bruise' to clear, it is often found at a later stage than melanoma on open skin.

It occurs across every skin tone. In people with brown and Black skin, melanoma appears more often on the palms, the soles, and under the nails than on sun-exposed skin, so a nail change deserves the same attention regardless of skin color — the skin-of-color guide covers where acral melanoma on soles, palms, and nails tends to show up. Because the other, more common skin cancers are rarer on this skin, a nail change can be an especially important early signal.

Two simple checks help sort out urgency. A true bruise should migrate outward and clear within a nail's growth cycle; one that stays put does not behave like a bruise. And while most nail melanomas grow slowly, a nail change that grows the way a fast nodular melanoma can — enlarging over weeks — is a reason to be seen sooner rather than to keep watching.

Photograph it and track it

Because no one can tell a benign band from a melanoma just by looking, the safe move is to document the nail and have it examined. A clear, dated photograph in good light — with a ruler or coin beside the finger for scale — gives a clinician a baseline and shows whether the band is widening. Photographing the same nail from the same angle every few weeks captures change that memory misses.

A few practical points make the record useful. Include the whole nail and the cuticle in the frame so the base and any spread of pigment are visible. Note the date each time, and track whether the pigment moves toward the tip — bruise-like behavior — or stays anchored at the base. It helps to leave the nail unpolished before a visit so nothing is hidden.

The same photograph-and-track method used for moles works here. But tracking is meant to inform the appointment, not to replace it. A band with concerning features should be seen even if one photo looks stable, because a single image cannot capture what only shows up over time, and a nail lesion cannot be diagnosed from a picture at all.

Toenails follow the same logic but are easier to neglect, partly because a stubbed toe makes a bruise a convenient explanation. A dark band on a big toe that cannot be tied to a specific injury, or one that persists past the months it would take to grow out, deserves the same photograph-and-examine approach as a fingernail. Good light and a steady hand matter even more here, since toenails are thicker and harder to see clearly.

How it is diagnosed and treated

A pigmented nail band that concerns a dermatologist is diagnosed with a biopsy of the nail matrix — the source of the pigment — not by appearance alone 4Ref 4American Family Physician (2011).Shave and Punch Biopsy for Skin Lesions.A suspected melanoma is diagnosed by biopsy, with full-thickness sampling preferred over superficial techniques to preserve accurate measurement of depth for staging.. For a suspected melanoma, the sample is taken full-thickness so a pathologist can measure how deeply it reaches, the number that drives everything afterward 4Ref 4American Family Physician (2011).Shave and Punch Biopsy for Skin Lesions.A suspected melanoma is diagnosed by biopsy, with full-thickness sampling preferred over superficial techniques to preserve accurate measurement of depth for staging.. Treatment of a confirmed subungual melanoma is surgical, with how much tissue is removed guided by the tumor's thickness 5Ref 5Swetter SM, Tsao H, Bichakjian CK, et al. (2019).Guidelines of care for the management of primary cutaneous melanoma.Confirmed melanoma is treated surgically, with excision margins guided by tumor thickness and sentinel lymph node biopsy considered for thicker tumors..

A nail biopsy is a specialist procedure, and it can temporarily or permanently change how the nail looks. That trade-off is exactly why clinicians reserve it for genuinely concerning bands rather than sampling every dark line — the goal is to catch the dangerous ones without scarring nails needlessly. It is usually done under local anesthesia in the office.

Historically, treatment of a confirmed nail melanoma meant amputating the fingertip. Today, narrower excisions are often used depending on how deep the melanoma goes, and a sentinel lymph node biopsy may be considered for thicker tumors to check whether cancer has reached nearby nodes 5Ref 5Swetter SM, Tsao H, Bichakjian CK, et al. (2019).Guidelines of care for the management of primary cutaneous melanoma.Confirmed melanoma is treated surgically, with excision margins guided by tumor thickness and sentinel lymph node biopsy considered for thicker tumors.. The plan is individual, and the thickness measured at biopsy — not the width of the streak you can see — is what shapes it and the outlook that follows.

How quickly to be seen

A new, changing, or widening pigmented band in a single nail, a band showing the Hutchinson sign, or a 'bruise' that has not grown out after a full nail-growth cycle is worth a prompt dermatology appointment — not an emergency, but not something to watch for months. A photograph sent through a clinician's teledermatology service can help sort out urgency, but a nail lesion usually needs an in-person exam and often a biopsy to settle it 6Ref 6American Academy of Dermatology (2024).Teledermatology Standards.Teledermatology can support triage of a submitted image, but concerning lesions are directed to in-person evaluation..

Fingernails grow slowly — a whole fingernail takes roughly six months to replace itself, and a toenail longer — so 'wait and see' can quietly turn into a long delay. Setting a short, specific recheck date, and keeping the dated photos to bring along, guards against that drift.

It is also worth separating two different ideas. Broad recommendations that question routine whole-body skin screening apply to people with no symptoms at all. A nail band you have already noticed changing is not screening — it is a symptom you have found, and evaluating a symptom is a different and clearer situation. The action stays the same whatever the base rate: document it, and have it looked at.

If a rough threshold helps: a pigmented band that is new in adulthood, confined to a single nail, and either widening or unexplained is the combination that most often earns a biopsy. But none of those has to be present for a nail that simply worries you to be worth a visit — the criteria narrow the decision, they do not license waiting.