Grey line along gum line?

Lately I have noticed a grey line along my front tooth's gum. I don't have any sensitivity and I take decent care of my teeth, with regular visits. It's not in a spot that I could floss, so I'm at a bit of a loss. About 9 months ago I did have some cosmetic dentistry done and they built up the tooth because it had been knocked back. But I haven't had any bridges or root canals or anything serious. What do you think is up?