You lean into the bathroom mirror and see a black spot on a back tooth, or a dark line near the gum. It does not hurt, but now you are wondering whether it is a cavity.
A black spot on a tooth can be one of several things, and only some of them need a drill. Let me walk you through what each one is, what tells them apart, and why the color is the weakest clue you have.
Is a black spot on your tooth always a cavity?
No. A dark spot usually turns out to be one of these seven things, and only the last one is a cavity that is still growing.
Surface stain. Coffee, tea, red wine and tobacco leave pigment in the narrow grooves of back teeth, where a toothbrush bristle cannot reach the bottom.
Black stain. This is a row of tiny black dots or a thin line along the gumline, most often in children. The suggested cause is bacteria that make hydrogen sulfide, which reacts with iron in saliva and gum fluid to leave a black iron salt. The evidence on its cause is still mixed (Alammari et al., 2024).
Tartar. This is plaque that has hardened, and above the gumline it is usually pale. Below the gumline it is usually dark brown, green or black (Balaji et al., 2019).
An old filling showing through. A silver filling is gray, and through a thin wall of enamel it can show as a dark shadow beside the filling.
A spot a dentist already treated. Silver diamine fluoride is a liquid that dentists apply to decay to stop it. The American Dental Association names black staining of the treated spot as a common side effect, and the black does not fade (ADA).
Decay that has stopped. Dentists call this arrested decay: it started, then conditions in the mouth changed and it stopped.
Decay that is still active. This is the cavity you were worried about.
Why can a darker spot be the safer one?
Decay starts when bacteria in plaque turn sugars into acid, and that acid pulls minerals out of the enamel. The National Institute of Dental and Craniofacial Research explains that early decay can show as a white spot where minerals have been lost. At that stage, minerals from saliva and fluoride can go back in and stop or reverse the decay (NIDCR). If more minerals leave than come back, the enamel breaks down into a cavity.
When the balance tips back toward repair, the spot stops growing and its surface hardens again. That surface can then pick up pigment from food and drink, which is one reason stopped decay can be brown or black.
The International Caries Classification and Management System, a framework dentists use to stage decay, describes the two side by side. A stopped spot can be whitish, brownish or black, and it feels hard and smooth when a probe slides gently across it. An active spot is whitish or yellowish, looks dull and feels rough (Ismail, Pitts and Tellez, 2015).
So the decay still eating into a tooth can be the pale, chalky spot. The American Dental Association's guideline on treating decay without drilling says hardness on probing is a sign decay has stopped (Slayton et al., 2018). Black color, it says, is not an acceptable way to judge that. A phone photo cannot feel hardness, so no photo can settle this.
What can you check at home?
None of these is a diagnosis, but each is worth telling us about.
Texture. Run your tongue over it. A smooth, glassy spot fits stain or stopped decay, while a rough patch, pit or ledge is worth pointing out.
Sensitivity. A quick twinge with sweets or cold can mean the enamel has a gap. Under the enamel is dentin, a softer layer with tiny channels that lead toward the nerve.
Floss. If floss shreds or catches in the same place every time, something there has an edge. It could be a cavity between two teeth, tartar, or the rim of an old filling.
Change. A spot that grows or changes shape over weeks matters more than one that has looked the same for years.
Pain is a poor guide. Decay does not hurt until it nears the nerve, so "nothing hurts" tells you how far it has traveled, not whether it is there.
What does an X-ray show that a mirror cannot?
The part that matters most sits under the surface. Dentin is softer than enamel, so decay spreads wider once it breaks through. A small dot on top can sit over a bigger hollow below.
The ICCMS has the dentist dry the tooth first, because the earliest enamel changes show only once the surface is dry. Then a probe moves gently across the spot to feel whether it is hard or soft. Gentle matters. A 2023 review warns that pressing a sharp tip into a softened spot can break a surface that could still repair itself.
An X-ray then shows how deep the decay reaches, and depth is one of the things that guides treatment. It has limits too. The same review cites research suggesting 30% to 40% of the enamel must lose its minerals before an early spot shows on an X-ray. The review also notes that an X-ray cannot tell active decay from stopped decay. The ICCMS authors add that two X-rays taken some time apart are normally needed to show whether a spot has grown.
That is what an exam and cleaning is for. We put the images on a screen and turn it toward you, so you can see what we are describing.
Does a dark spot mean you need a filling?
Not always, and the answer depends on which of the seven it turns out to be.
Stain, black stain and tartar come off at a professional cleaning, though the black stain paper notes that it often comes back. Tartar below the gumline can call for a deep cleaning.
Stopped decay that is hard and easy to keep clean can often be left alone and checked again later. For early decay, the ICCMS recommends sealants, fluoride and better home care rather than drilling. If decay is still within the enamel, we will tell you when fluoride and better cleaning are the right answer rather than a drill.
Active decay that has broken into the dentin usually needs a filling. We place tooth-colored composite fillings, which bond to the tooth, so only the decayed part has to come out.
A sound silver filling does not need replacing just because it looks dark, although decay under its edge is a different matter.
Why not fill every dark spot to be safe? Drilling a stopped spot removes healthy tooth to fix a problem that had already ended. The filling then has an edge of its own, and decay can start again under that edge.
If a spot does need treatment, you get what we found written down before you agree to anything, and the decision is yours.
When should you call us about a black spot?
Call sooner rather than waiting for your next checkup if any of these apply:
- The spot has grown, changed shape or turned into a pit
- You feel a twinge with sweets or cold on that tooth
- An ache lingers after the cold or sweet is gone
- Floss keeps shredding in the same place
If there is swelling in your face or gum, see our dental emergency page. If nothing has changed and nothing hurts, point it out at your next visit.
Call Advanced Dental Care of Stafford at (540) 288-2800 or send us a message.
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