A brown, gray or black line around the edge of a filling is one of the most common things people notice in the mirror. Often it is harmless stain that has settled into a tiny ledge at the edge. Sometimes it means decay has started underneath. You cannot reliably tell the two apart by looking, but a dentist usually can in a few minutes.
Published September 2026
Why a line forms at the edge
Where a filling meets the tooth there is a join. When the filling is new, the join is smooth and sealed. Over the years the filling wears, the tooth flexes slightly every time you chew, and the join can develop a tiny step or gap. Stain from coffee, tea, red wine and tobacco collects there, which is why the line shows up around the edge rather than across the whole filling.
The four usual causes
Surface stain at the edge. The most common and the least worrying. The seal is intact, and the line is stain sitting in a slightly rough edge. It can usually be polished away, or the edge smoothed and sealed.
A leaking edge. The seal has opened up enough for stain to get under the edge of the filling. There may be no decay yet, but bacteria can now reach the tooth, so the edge needs repairing or the filling replacing.
Decay under the filling. Also called recurrent or secondary decay. The line is soft, decayed tooth rather than stain. It will not stop on its own, and the filling has to be replaced before the decay gets close to the nerve.
Gray from an old silver filling. Silver (amalgam) fillings can slowly stain the tooth around them gray, and a tooth-colored filling placed over or next to an old silver one can show a gray shadow. This is a color problem rather than a health problem, though it is worth checking that the tooth is sound.
Signs it is more than stain
- the line has appeared or darkened quickly
- the tooth is sensitive to cold, sweet things or biting
- floss catches or shreds at the edge of the filling
- you can feel a step or a gap with your tongue
- the filling is chipped or cracked
None of these is an emergency on its own, but they are worth a visit soon rather than waiting for a check-up months away.
How we tell the difference
We look at the edge under a bright light, check with a fine instrument whether it is hard and sealed or soft and catching, and take an x-ray where decay could be hiding between teeth or under the filling. Most of the time it is clear within a few minutes which of the four you have.
What fixes each one
- Surface stain: polishing, often at a routine cleaning.
- A rough or slightly open edge: smoothing and resealing the edge, or repairing it with a small amount of new composite, without replacing the whole filling.
- Decay or a failed seal: replacing the filling. If the tooth has been weakened, a crown may protect it better than a larger filling.
- Gray from old silver: replacing the silver filling with a tooth-colored filling. Some gray staining in the tooth itself can remain, and where that matters on a front tooth, bonding or a veneer covers it.
Catching decay at the edge of a filling early usually means a straightforward replacement. Left for years, the same decay can reach the nerve and turn a filling into a root canal.
If the discoloration is spread across the whole filling rather than around its edge, see why fillings stain or turn yellow.
Common questions
Why is there a dark line around my filling?
Usually stain that has collected where the filling meets the tooth, as the edge wears slightly over the years. Less often it is a leaking seal or decay under the filling, which is why a line that is new, spreading or sensitive should be checked.
Is a black line around a filling decay?
Not always. Stain and decay can look similar. A hard, sealed edge with a stain line is usually cosmetic; a soft or catching edge, sensitivity, or a line that is spreading suggests decay. An examination, and sometimes an x-ray, tells them apart.
Why is my white filling turning gray or black?
A composite filling itself tends to yellow rather than blacken. Gray or black usually comes from stain at the edge, decay underneath, or an old silver filling showing through or staining the tooth nearby.
Can the stain around a filling be removed?
If it is surface stain on a sealed edge, yes, it can usually be polished away. If the stain has got under the edge, the edge needs repairing or the filling replacing.
Does a dark line mean the filling has to be replaced?
Not necessarily. Many can be polished or repaired. Replacement is needed when there is decay underneath or the seal has failed.
Noticed a line around a filling? Call 201-425-8600 or request an appointment. We are at 115 Christopher Columbus Drive, Suite 200, two minutes from the Grove Street PATH. More articles and treatment guides are on our patient guides page.