What is the dark line at the base of my tooth near the gum?

A dark line at the base of your tooth near the gum is most often caused by one of seven things: tartar buildup, a gumline cavity, gum recession exposing the root surface, the metal edge of a dental crown or surface staining from food and drink. Most causes are treatable — and some are purely cosmetic — but a professional evaluation is the only reliable way to know which one you are dealing with.

What you'll learn in this article:

• The seven most common causes of a dark line at the gumline — and how to tell them apart

• The key difference between a surface stain and something that needs treatment

• Which symptoms mean you should not wait to be seen

• What your Aspen Dental provider will look for during an evaluation

What causes a dark line at the base of your tooth?

The location, texture, and appearance of the line — and whether it is on a natural tooth or a crowned tooth — all point toward different explanations. Here are the seven most common causes.

Tartar buildup

Tartar — also called calculus — is hardened plaque that has mineralized on the tooth surface. Above the gumline, tartar typically appears yellow or tan. At or below the gumline, it can turn dark brown or black as it absorbs pigments from food, drink, and bacteria over time. Tartar cannot be removed with brushing alone — it requires a professional cleaning.

What it looks like: A dark, rough-edged line or band that follows the curve of the gumline, often across multiple teeth. It may feel rough if you run your tongue across it.

Gumline cavity (root-surface decay)

A gumline cavity forms where the tooth meets the gum — an area that is harder to clean and more vulnerable to decay, especially when gums have receded and exposed the softer root surface. Unlike enamel, the root surface is covered by cementum, which is less resistant to acid and decay.

What it looks like: A dark spot or line that may appear discolored and feel slightly rough or soft to the touch. Unlike a stain, a gumline cavity often causes sensitivity to cold, sweet foods or pressure. It can progress quickly if left untreated.

Gum recession exposing the root

When gum tissue pulls back from the tooth — due to gum disease, aggressive brushing or natural aging — it exposes the root surface below. The root is naturally darker than the enamel crown of the tooth, which creates the appearance of a dark line or shadow at the gumline. The line itself is not decaying. It is the color difference between enamel and cementum becoming visible.

What it looks like: A distinct color shift at the gumline, often yellowish-brown or darker than the rest of the tooth. The tooth may also appear longer than it used to.

The metal edge of a dental crown

If you have a porcelain-fused-to-metal (PFM) crown — a common crown type used for decades — the metal substructure beneath the porcelain can create a dark line at the gumline. This happens when gum tissue recedes slightly over time, exposing the metal margin of the crown.

What it looks like: A sharp, precisely defined dark line that runs exactly along the crown margin. It is most noticeable on front teeth. This is primarily a cosmetic concern — it does not mean the crown has failed — but it can be addressed by replacing the crown with an all-ceramic alternative.

Surface stains

Coffee, tea, red wine, tobacco, and certain foods leave dark deposits on the tooth surface, particularly at the gumline where plaque tends to accumulate. These stains sit on the outer surface of the enamel and do not indicate decay or structural damage.

What it looks like: A diffuse, uneven darkening that may affect multiple teeth. Surface stains typically polish off with a professional cleaning and do not cause sensitivity.

Abfraction lesions

Abfraction lesions form when repeated stress on a tooth — from grinding, clenching or aggressive brushing — causes the enamel at the gumline to thin, chip or wear away. As the enamel thins, the darker dentin layer underneath becomes visible, creating a line or notch at the base of the tooth that can look brown, grey, or black. Unlike decay, abfraction is a mechanical wear issue, not a bacterial one — but the exposed dentin it creates can become vulnerable to sensitivity and, over time, decay if left unaddressed.¹

What it looks like:

A small wedge-shaped notch or groove right at the gumline, often on the side of the tooth facing the cheek. The area may feel slightly rough or indented when you run your tongue across it. It is most common on teeth that take the most biting force — premolars and canines — and often appears on multiple teeth rather than just one.

Chromogenic bacteria

Some people develop dark lines at the gumline not because of decay, tartar, or wear — but because of specific bacteria that naturally produce dark pigment as a byproduct of their activity. These are called chromogenic bacteria, and they create thin, dark deposits that adhere to the tooth surface at or just above the gumline. What makes this cause particularly confusing for patients is that it can appear even in people with excellent oral hygiene — the staining is not a sign of poor brushing habits.²

What it looks like:

A thin, consistent dark line running along the gumline of one or more teeth, often appearing black or very dark brown. The line tends to be smooth rather than rough, does not cause sensitivity, and may return after a professional cleaning because the bacteria responsible are still present. If the line reappears consistently after cleanings, chromogenic bacteria are a likely cause.

How to prevent dark lines at the gumline?

Most dark lines at the gumline are not inevitable. Four habits address the majority of causes — from tartar buildup and gum recession to surface staining and early decay.

Brush along the gumline, not just across the teeth.

Angle your toothbrush at 45 degrees toward the gum and use short, gentle strokes. Aggressive horizontal scrubbing is one of the leading causes of gum recession and abfraction lesions — both of which create the conditions for dark lines to form. A soft-bristled brush is non-negotiable.

Get a professional cleaning every six months.

Tartar — one of the most common causes of dark gumline buildup — cannot be removed by brushing alone. It requires professional scaling. Patients who skip cleanings allow calculus to accumulate in the exact location where dark lines are most visible and most likely to cause damage.

Limit foods and drinks that stain and erode enamel.

Coffee, tea, red wine, and dark sodas contribute to surface staining at the gumline. Acidic foods and drinks — citrus, vinegar-based dressings, carbonated beverages — thin enamel over time, making the darker dentin layer underneath more visible. Rinsing with water after consuming these does not eliminate the effect but meaningfully reduces it.

Address grinding and clenching before it causes visible damage.

Bruxism is a direct cause of abfraction lesions — the wedge-shaped notches at the gumline that expose dentin and create dark lines. If you wake up with jaw soreness, headaches, or tooth sensitivity, mention it at your next appointment. A custom mouthguard can stop the mechanical wear before it becomes visible.

Is this dark line a stain or something I should get checked?

The most important distinction is between a surface stain — cosmetic, no treatment needed beyond a cleaning — and something that requires professional care. Use these two scenarios as a guide, not a diagnosis.

It is more likely a surface stain if:

• The line is smooth and does not catch on floss or your fingernail

• It causes no sensitivity to cold, heat or sweet foods

• It appears across several teeth rather than one specific spot

• It has developed gradually and you link it to coffee, tea, tobacco, or red wine

Surface stains typically respond well to professional cleaning and do not indicate any structural damage to the teeth.

It is more likely something that needs treatment if:

• The line feels rough or slightly soft when you run your tongue across it

• It is concentrated on one tooth or one specific area

• It causes sensitivity to cold, sweet foods or pressure

• It has appeared or changed noticeably over a short period

A dark line that is rough, localized or accompanied by any sensitivity warrants a professional evaluation — regardless of how minor it looks.

When to see your Aspen Dental provider

See your Aspen Dental provider promptly if the dark line:

• Is accompanied by sensitivity to cold, heat or sweet foods

• Feels rough or soft when you run your tongue across it

• Has appeared or grown noticeably over a short period

• Is concentrated on a single tooth rather than spread across several

• Is accompanied by soreness, throbbing, or tenderness in the area

• Comes with bleeding or swollen gum tissue nearby

Schedule a routine appointment if the dark line:

• Has been present for a long time without changing

• Causes no sensitivity and feels smooth to the touch

• Appears across multiple teeth near the gumline

• You suspect it may be tartar or staining

Even if the line turns out to be cosmetic, a professional cleaning and exam will confirm the cause and give you a clear picture of your gum health.

What to do next

A dark line at the base of your tooth is worth having evaluated — not because it is always serious, but because the causes that do require treatment respond much better to early care than delayed care. A gumline cavity caught early may need only a small filling. Left untreated, it can reach the root and require more extensive treatment.

Your Aspen Dental provider can examine the area, identify the cause, and explain your options clearly — without guesswork.

Schedule an appointment with your Aspen Dental provider today → Same-day and next-day appointments are available at many locations. No referral needed.

Dark line at the base of tooth FAQs

Is a dark line at the gumline always a cavity?

No. A dark line at the gumline can be caused by tartar buildup, surface staining, gum recession or the metal edge of a dental crown — none of which are cavities. However, a gumline cavity is one possible cause and can look similar to a stain. A professional exam is the only reliable way to tell the difference.

Can I remove a dark line at the gumline at home?

It depends on the cause. Surface staining may improve with whitening toothpaste, but a professional cleaning is more effective. Tartar cannot be removed at home — it requires professional scaling. If the line is a cavity, crown margin or gum recession, home care will not resolve it. Do not attempt to scrape or pick at the area.

Does a dark line at the gumline mean I have gum disease?

Not necessarily. Tartar buildup at the gumline is a risk factor for gum disease, but its presence alone does not confirm a diagnosis. Gum recession — which can expose the darker root surface — can result from gum disease, but also from aggressive brushing or natural aging. Your Aspen Dental provider can assess your gum health during an exam.

Why does my tooth look darker near the gum than at the top?

The upper portion of your tooth is covered by enamel, which is naturally lighter and more translucent. Near the gumline and below, the tooth is covered by cementum — a softer, naturally darker material. When gums recede even slightly, this color difference becomes visible and can look like a dark line or shadow.

Will the dark line go away on its own?

It depends on the cause. Surface stains can be removed with a professional cleaning. Tartar requires professional scaling. Gumline cavities, crown margins and gum recession do not resolve on their own and require professional treatment.

Is a dark line on a crowned tooth serious?

A dark line on a crowned tooth is most often the metal margin of a porcelain-fused-to-metal (PFM) crown becoming visible as the gum recedes slightly. This is primarily a cosmetic issue and does not mean the crown has failed. If it bothers you aesthetically, your Aspen Dental provider can discuss replacing it with an all-ceramic crown.

Sources

¹(NIH/PMC, Nascimento et al., Abfraction lesions: etiology, diagnosis, and treatment options) https://pmc.ncbi.nlm.nih.gov/articles/PMC4861607/

² NIH/PMC scoping review on chromogenic bacterial staining of teeth (January 2025). Approved source. https://pmc.ncbi.nlm.nih.gov/articles/PMC11725193/