Why is there a black line showing on the 6-months old zirconia crown? (Photo)

There can be three things that can cause a dark line at a crown margin.  First, since the tooth had a root canal, the entire remaining part of the tooth will turn darker over time. This can show through the thin tissue sometimes especially where the crown margin meets the tooth.  Also, the crown can cast a shadow and make the root portion seem darker.  If you have some gum recession, this area will show more readily.  Finally, if there is a small space between the crown margin and the part of the tooth that was prepared for the crown, decay can form there making it dark.  Since it has only been in place for 6 months, the decay reason is unlikely.  I would recommend seeing your dentist and let him see what you are concerned about.

The underlying color of your tooth may be giving off this darker shade. It is unlikely going to affect the long term prognosis of the crown but I would inform your dentist about the concern

Is the dark black line seen on the root or does the gums appear darker around the crown?It may be that your gums have receded slightly, exposing an underlying dark root.  Dark roots are often because a tooth died and discolored before having a root canal, or a tooth that had a root canal and have a metal post that darkens both the root and the crown.  Many times in highly esthetic regions, especially the front teeth, cosmetic dentists will remove metal posts before doing new all-porcelain crowns and instead, place tooth-colored fiber posts (strong like Kevlar, used in bullet proof vests) and tooth-colored composite core.Also, all ceramic porcelains like Empress and EMax generally look more natural that the more opaque zirconia crowns, however zirconia is much stronger.You may be interested in going to this web link regarding cases I have treated that had black lines around the gums.