Quick dentist question please re. 17 month old

[SleepFreeZone]

He has beautiful little white teeth. He is yet to cut his final four at the back. Today I noticed when he was whinging and grizzling (so had his mouth open wide) that he has a black line where the underside of one of his front teeth meet his gum line.

I just gave his teeth a good brush to check it wasn't good debris or dirt, but it's still there!

He has seen the dentist twice for checks and she's always happy. Should I be taking him back to get it checked out? She doesn't really have a good look so I'm not sure he'll let her stick a mirror in there without chomping on it 😬

Why not phone the dentist for advice? I am sure she will be happy to either say over the phone whether it's something to worry about or will say if it's better to bring him in for a quick look.

Just to add he is still breast fed twice a day and only drinks milk and water out of one of those sip cups that are good for teeth.

I just Googled and found on this page

Some populations naturally form a black line on the teeth at the gums lines of all the teeth. It tends to reform rather quickly after removal by the dentist. Where it comes from we do not know. But, we do find that these patients seem to develop few dental cavities.

So sounds like it's not a bad thing

I read something similar on googling but that just sounds like nonsense to me 😬