A black or grey line running along the gum edge of a crown is almost always the metal margin of an older porcelain-fused-to-metal crown becoming visible, usually because the gum has receded slightly over the years or the metal underneath has oxidised. In most cases it is a cosmetic issue rather than a dental emergency. The exception is when the dark line comes with pain, swelling or a bad taste, which points to something that does need attention.
Key Takeaways
- The most common cause is a metal margin on a porcelain-fused-to-metal crown becoming visible as gums recede slightly with age.
- Metal oxidation over years of wear can also darken the margin, even without noticeable gum movement.
- A black line on its own, with no pain, swelling or looseness, is usually cosmetic rather than a sign of a health problem.
- A dark line paired with tenderness, swelling, bad taste, or a visible gap between the crown and tooth can point to decay or active gum disease.
- Modern all-ceramic and zirconia crowns have no metal core, so they do not produce this dark line even if gums recede later.
- An existing black line can usually be corrected by replacing the crown with a metal-free option, without necessarily needing work on the tooth underneath if it is otherwise healthy.
What causes the black line
Metal margins on older crowns
Porcelain-fused-to-metal crowns, often shortened to PFM, are built on a metal substructure with porcelain layered over the top for a tooth-like appearance. Near the gumline, some older techniques leave a thin band of metal exposed or only lightly covered, because that margin needed to sit precisely against the tooth. For years this metal edge can sit just under the gum, invisible. As the gum naturally recedes with age, or with gum disease, that hidden metal edge is gradually exposed and shows through as a dark line.
Gum recession over time
Gum recession itself has several causes unrelated to the crown, including brushing too hard, thin gum tissue, grinding, and general gum disease. Recession around a PFM crown does not always mean the crown itself caused the problem. It often just reveals a margin that was always going to become visible eventually.
Metal oxidation
Some metal alloys used in older crowns can also darken slightly over time through oxidation, a bit like tarnishing, which deepens the grey or black appearance at the margin even without much gum change.
Cosmetic issue or something more serious
The line itself is rarely dangerous, but the symptoms around it matter. Use the table below as a general guide, and see a dentist for anything in the right-hand column.
| Sign | Usually cosmetic | Worth a dental check |
|---|---|---|
| Dark line at gum edge only | Yes, if no other symptoms | - |
| Tenderness or swelling nearby | - | Yes |
| Bad taste or odour from the area | - | Yes, may indicate decay or infection |
| Visible gap between crown and tooth | - | Yes, crown may need replacing |
| Line has been stable for years | Yes | - |
| Line has appeared or darkened recently | - | Worth checking, especially with other symptoms |
Why modern crowns avoid this issue
All-ceramic and zirconia crowns are made without a metal substructure, so there is no metal margin to become visible if the gum recedes later in life. This is one of the main reasons zirconia and other all-ceramic materials have become the default choice for visible teeth in modern dentistry, alongside their strength and closer match to natural tooth translucency. We compare the materials in more detail in our guide to zirconia vs porcelain-fused-to-metal crowns.
How an existing black line is fixed
If the tooth and gum are otherwise healthy, the most straightforward fix is replacing the crown with an all-ceramic or zirconia crown, which removes the metal margin entirely. Where a dentist wants to keep an existing well-fitted PFM crown, some cases can be improved with careful margin recontouring or gum-level adjustments, though replacement is generally the more predictable long-term fix. If the black line is joined by decay or gum disease, that underlying issue needs to be addressed first, since a new crown placed over an unresolved problem will not solve it.
If you are noticing sensitivity as well as discolouration around a newer crown, that is usually a separate and temporary issue covered in our article on whether it is normal for a new crown to be sensitive to cold. If your crown has come loose or fallen off entirely, see our guide on what to do when a crown falls off.
Frequently asked questions
Is a black line around my crown dangerous?
On its own, usually not. It is typically a cosmetic sign of an ageing PFM crown rather than a health risk, but it is still worth mentioning at your next check-up.
Can I get rid of the black line without replacing the whole crown?
Sometimes minor adjustment is possible, but in most cases replacing the crown with an all-ceramic or zirconia option gives the most reliable, long-lasting result.
Does gum recession mean my crown was done badly?
Not necessarily. Gum recession has many causes unrelated to crown placement, including brushing habits, thin gum tissue and general gum health, and it can happen around any crown over enough time.
How do I know if it is decay and not just the metal showing?
Decay is more likely to come with sensitivity, a soft or rough feel to the area, or a visible gap or dark spot that looks different from a straight, even line at the margin. A dental check is the only reliable way to tell.
Will a new crown definitely avoid this problem?
All-ceramic and zirconia crowns remove the specific cause of this dark line, since there is no metal margin to become visible. Gum recession can still occur for other reasons, but it will not create the same dark shadow.
How much does replacing a PFM crown with zirconia cost?
Cost varies with the tooth, complexity and any additional work needed. A dental exam and quote is the only accurate way to know your specific cost.
If a black line at the gum is bothering you, or you would simply like a second look at an older crown, the team at Lumi Dental offers a complimentary cosmetic consult to talk through your options honestly, including whether replacement is worthwhile. Book via our free cosmetic consult page or see current offers.
