Black discoloration under or on your nails usually comes from one of a few common causes: a bruise from trauma, a fungal or bacterial infection, external staining, or (less commonly) a pigmented streak that needs medical evaluation. Most cases are harmless and grow out on their own, but the appearance and pattern of the discoloration can help you figure out what you’re dealing with.
Bruising Under the Nail
The most common reason for a dark spot under a nail is a bruise, known medically as a subungual hematoma. Stubbing your toe, dropping something on your finger, or even wearing tight shoes can damage the tiny blood vessels in your nail bed. When those vessels leak, blood pools in the tight space between the nail bed and the nail plate. Since there’s nowhere for the blood to go, it gets trapped and shows through as a dark red, purple, or black spot.
These bruises are often painful right after the injury, though sometimes the trauma is so minor you don’t remember it happening. The discoloration gradually moves toward the tip of your nail as it grows. Fingernails grow about 3.5 mm per month, so a bruise near the base of a fingernail can take six to nine months to fully grow out. Toenails are even slower at roughly 1.6 mm per month, meaning a black toenail from a running shoe that’s too snug could stick around for over a year.
If a nail bruise is large and throbbing, a doctor can relieve the pressure by making a small hole in the nail plate to drain the trapped blood. This works best within 24 to 48 hours of the injury. After that window, the blood has already started to clot and the procedure is less effective.
Fungal and Bacterial Infections
Nail infections don’t always look like the yellowish, crumbly nails you might picture. Certain fungi and bacteria produce dark pigments that turn the nail greenish-black or solid black. One common culprit is a type of yeast called Candida parapsilosis, which can cause greenish-black discoloration along with the nail lifting away from the bed. This is more common in people with weakened immune systems, but it can happen in otherwise healthy adults too.
Bacterial infections, particularly from Pseudomonas aeruginosa, produce pigments that stain the nail green to black. This bacterium thrives in moist environments, so you’re more likely to pick it up if your hands are frequently wet, if you wear artificial nails that trap moisture, or if a nail is already damaged. Unlike a bruise, an infection tends to spread gradually and may involve nail thickening, crumbling, or a foul smell.
External Staining
Sometimes the black stuff on your nails is exactly that: surface staining from something you touched. Hair dye, nicotine, potassium permanganate (used in some water treatments and skin remedies), certain skin-lightening creams containing mercury or hydroquinone, and even photographic chemicals can all leave dark deposits on or under the nail plate. Gardening in dark soil, handling certain metals, or using products containing iron compounds can do the same.
The key difference with external stains is that the discoloration sits on or near the surface rather than deep in the nail bed. You can sometimes scrub or buff it off. If the stain has soaked into the nail plate, it will grow out over the next few months as the nail replaces itself.
Splinter Hemorrhages
If the black marks look like tiny vertical lines running lengthwise under your nail, almost like splinters, they’re likely splinter hemorrhages. These are small streaks of blood from damaged capillaries in the nail bed. A single splinter hemorrhage is usually from minor trauma and is nothing to worry about.
Multiple splinter hemorrhages across several nails, especially if you haven’t injured them, can occasionally signal something deeper. Between 15% and 33% of people with endocarditis (an infection of the heart valves) develop splinter hemorrhages, and up to 35% of people with lichen planus (an inflammatory skin condition) report them. If you’re seeing these lines on multiple nails without a clear cause, it’s worth having them checked.
Melanonychia and Pigmented Streaks
A brown or black stripe running the length of your nail, from the base to the tip, is called melanonychia. This happens when pigment-producing cells in the nail matrix (the tissue under the cuticle where the nail grows) become active. It can be triggered by normal causes like pregnancy, vitamin B12 deficiency, or simply having darker skin, where longitudinal pigmented bands are extremely common and benign.
However, a new or changing dark streak deserves attention because it can, in rare cases, be a sign of melanoma under the nail. Dermatologists use a set of criteria to evaluate whether a streak is concerning:
- Age and ancestry: Higher risk between ages 50 and 70, and in people of African, Japanese, Chinese, or Native American descent
- Band width: Wider than 3 mm with an irregular or blurred border
- Change: A streak that is growing wider, darkening, or changing shape
- Digit: The thumb, big toe, and index finger are higher-risk locations
- Extension: Pigment spreading beyond the nail onto the surrounding skin (called the Hutchinson sign), which is a strong indicator of melanoma
- Family history: A personal or family history of melanoma
If a streak raises suspicion, a dermatologist will take a small biopsy of the nail matrix, either with a punch tool or a scalpel, to examine the cells under a microscope. Catching nail melanoma early makes a significant difference in outcomes, so a streak that checks more than one of the boxes above is worth showing to a dermatologist sooner rather than later.
How to Tell the Difference
A few simple observations can help you narrow down what’s going on. A bruise tends to be a single, round or irregular blotch that moves toward the tip of your nail as it grows. You can usually trace it back to an injury. An infection typically involves changes in nail texture (thickening, crumbling, lifting) and may worsen over weeks. External stains are usually on multiple nails, especially on the dominant hand, and correlate with something you’ve been handling.
A pigmented streak, by contrast, is a consistent line running base to tip that doesn’t move with nail growth because the pigment is being continuously produced at the matrix. If you notice a new streak that’s wider than a few millimeters, has blurry edges, or seems to be darkening or spreading onto the skin around the nail, that pattern is the one worth getting evaluated promptly.