Why does my dental crown have a black line by the gumline?

So you are looking into your mouth and you notice a black line at the base of your dental crown by the gum line.  What is it?  Basically it could be one of two things: 1) it could be a cavity forming or 2) your gums are receding and the margin of your dental crown which is made of metal is showing.

There are a few types of dental crowns.  Two types are shown in the picture.  The one on the left is an all ceramic crown.  The one on the left is a porcelain fused to metal ( PFM crown).  A PFM crown has a metal substructure and then porcelain baked over it so it looks like a tooth.

The only solution if you don't like the black line showing is to have a new crown fabricated, preferably an all ceramic crown that does not have a metal substructure.  There is also another type of PFM crown that is called a Captek crown in which the metal substructure is gold.  So instead of a black line you will see a yellow line which blends in much better.  Captek crowns tend to be more expensive because gold is expensive.