A black line on a nail is usually a benign splinter hemorrhage or a streak of melanin pigment called longitudinal melanonychia, and most single dark bands are harmless. The urgent exception is subungual melanoma, a skin cancer under the nail that accounts for roughly 0.7 to 3.5 percent of all melanoma cases worldwide. The practical rule: a new dark band that widens, darkens, or involves the surrounding skin needs a dermatologist's eyes, ideally within 4 to 6 weeks. The sections below separate the roughly 20 known causes, show how pigment behaves over months, and explain the Hutchinson sign that changes urgency.
What Actually Produces a Dark Band
Nail plates are translucent keratin, roughly 0.5 mm thick on the fingers and up to 1.0 mm on the toes. Color seen through them comes from structures beneath. A black line therefore has only two broad anatomical sources: blood trapped between nail layers, or melanin deposited in the nail matrix, the tissue at the cuticle that manufactures the plate.
Splinter hemorrhages arise when tiny capillaries running lengthwise beneath the plate rupture. Blood tracks along those grooves, producing thin lines 1 to 2 mm long, often near the free edge. Trauma accounts for most of them, including repetitive microtrauma from tight shoes or typing.
Melanonychia works differently. Melanocytes in the matrix activate and release pigment into the growing nail, so the band moves outward with the plate at about 0.1 mm per day on fingers. That growth rate means a band beginning at the cuticle takes roughly 4 to 6 months to reach the tip of a fingernail.
In plain terms: blood lines are leaks under a window; melanin bands are paint fed into the glass from below.
The Causes Ranked by Frequency
Longitudinal melanonychia dominates in adults with darker skin tones. A 2015 review of 1,001 nail biopsies found melanoma in only 2.3 percent of pigmented bands overall, but the proportion rose sharply in patients over 50 and in bands wider than 6 mm.
Benign melanonychia from ethnic pigmentation affects an estimated 77 percent of African American adults over age 20, 40 percent of Hispanic adults, and 20 percent of Asian adults. It usually involves multiple nails and appears symmetrically.
Subungual hematoma, blood pooling after a crush injury, is the most common single-nail cause in people of any skin tone. It typically presents as a round dark spot rather than a line and migrates outward.
Less common causes include:
- Fungal infection (onychomycosis), which can produce brown-black discoloration in roughly 50 percent of dark-variant cases and often thickens the plate
- Medications, especially chemotherapy agents, that trigger matrix pigmentation in 10 to 20 percent of treated patients
- Psoriasis and lichen planus, inflammatory conditions that disturb matrix function
- Nutritional deficiency, mainly vitamin B12, which produces diffuse rather than single-nail darkening
- Endocrine disease such as Addison's disease or thyroid dysfunction
- Benign matrix tumors, including nevi, that behave like moles under the nail
A Quantified Comparison of the Main Causes
The table below reconciles the features clinicians weigh. Width thresholds are the most debated: the classic 6 mm cutoff for melanoma suspicion has a sensitivity near 60 percent, so width alone cannot rule cancer out.
| Feature | Splinter hemorrhage | Benign melanonychia | Subungual hematoma | Subungual melanoma |
|---|---|---|---|---|
| Typical shape | Thin line, 1-2 mm | Even band, 1-3 mm | Round spot, 3-10 mm | Irregular band, often over 6 mm |
| Number of nails | 1-3 | 4 or more | 1 | 1 |
| Color change over time | Fades in 2-4 weeks | Stable or slow | Migrates, fades | Darkens, widens |
| Skin around nail | Normal | Normal | Normal | Hutchinson sign in 30-60 percent |
| Growth pattern | Does not move with the nail | Moves outward with plate | Moves outward | Moves outward, widens |
| Age group most affected | Any | Over 20, darker skin | Any, after injury | Peak at 50-70 |
Melanoma risk climbs with age. In one 2019 series of 148 subungual melanoma cases, the mean age at diagnosis was 63 years, and 68 percent involved the great toe or thumb.
A Month-by-Month Timeline
Pigment behavior over time is the single most useful observation. A band that changes shape within weeks behaves very differently from one that stays identical for two years.
| Timeframe | What benign causes do | What melanoma tends to do |
|---|---|---|
| Week 1-2 | Hematoma darkens, then stabilizes | Band appears suddenly, often darker |
| Week 3-4 | Splinter hemorrhage begins to fade | Band shows no fading |
| Month 2-3 | Hematoma migrates toward the tip | Band continues to widen past 6 mm |
| Month 4-6 | Nail grows out, discoloration clears | Hutchinson sign may appear on skin |
| Month 6-12 | Stable ethnic bands stay identical | Nail may separate from the bed |
| Year 1-2 | No structural change | Nail plate can crack or distort |
| Year 2+ | Benign nevi change very slowly | Metastasis risk rises with delay |
The plate growth rate of 0.1 mm per day means a 10 mm band takes about 3 to 4 months to travel the visible fingernail. If a band sits at the same position for longer than that, it is not progressing normally.
The Hutchinson Sign and Other Red Flags
The Hutchinson sign describes pigment spreading from the nail onto the cuticle or skin beside it. It appears in 30 to 60 percent of subungual melanoma cases, so its absence does not exclude cancer, and its presence strongly favors biopsy.
Seek evaluation within 2 to 4 weeks if any of the following appear:
- A single dark band wider than 6 mm
- Pigment touching the cuticle or surrounding skin
- A nail that splits, bleeds, or separates from the bed
- A band that darkens or widens over 6 to 8 weeks
- A dark line in a child or in a nail without any injury history
- Pain, ulceration, or a visible mass under the nail
The counterintuitive caveat: a narrow, faint band in a 30-year-old with identical streaks on several nails is very likely ethnic pigmentation and needs no biopsy. Over-referral of harmless bands is common, and dermatoscopy with a nail plate examination reduces unnecessary surgery substantially.
Why Misconceptions Persist
The biggest misconception is that any black line is cancer. The biopsy-proven rate is close to 2 to 4 percent of pigmented bands, so the base rate is low. A second misconception treats nail removal as a benign fix. Most subungual melanomas involve the matrix, so partial nail avulsion can miss the lesion entirely; the standard procedure is a longitudinal matrix biopsy, and prompt biopsy remains the diagnostic gold standard per the American Academy of Dermatology's 2023 melanoma guidelines.
A third error confuses nail streaks with systemic disease. Splinter hemorrhages were historically linked to endocarditis, but a 2020 analysis found they were present in fewer than 5 percent of confirmed cases and were less predictive than fever or heart murmur. In practice, isolated nail streaks rarely signal infection.
What to Actually Do
Photograph the nail today with a ruler or coin for scale and repeat monthly under identical lighting. This creates an objective record that beats memory.
Track two numbers: band width in millimeters and distance from the cuticle. If either changes by more than 2 mm over 8 weeks, book a dermatology visit.
Do not paint over the nail with polish. Wait for the natural nail to be examined first; polish hides the band and can delay assessment by weeks.
For suspected hematoma, allow 4 to 6 months of nail growth before assuming the spot is permanent. Most resolve on their own in that window.
Avoid aggressive cuticle cutting. The cuticle protects the matrix, and matrix injury can itself trigger pigmentation and infection.
Limits of the Evidence and Standing Uncertainties
Nail pigmentation research is thin. Most recommendations rest on retrospective case series rather than randomized trials, and the 6 mm width threshold comes from studies of fewer than 200 patients. No blood test reliably distinguishes benign from malignant bands. Dermatoscopy, which improves accuracy in studies, still depends on operator skill and has a documented learning curve of 50 to 100 cases before clinicians reach 85 percent accuracy. For anyone with an evolving or uncertain nail band, biopsy remains the only definitive answer, and the cost of a 10-minute matrix sampling procedure is far lower than the cost of a delayed melanoma diagnosis.