Based on your photos and your description, it sounds like you are having some bacteria accumulation around your gum margins. This is most often due to poor oral hygiene or a non dental friendly diet. I would suggest getting regular teeth cleaning by a dental hygienist every 6 months,...
Based on your description, it sounds like you have only a crown prep remaining, in which looks like a short tooth. If this is the case, it is not difficult at all to re-prep the tooth and place another crown. It would be in your best interest to take care of this tooth very soon,...
If you have a dark line around your crown, it sounds like you have a metal crown that is coated with porcelain. If this is the case, it is impossible to remove that line without replacing the crown. I would recommend getting a full porcelain emax crown on this tooth to get rid of...
