What is the thin grey line I see above (or below) my crown?

Usually when I am asked this by a patient I find is they have a porcelain fused to metal crown that has been finished on the outside with a thin metal collar. This is usually done to create a finer margin for the restoration and can help insure that the crown fits optimally and the porcelain doesn't get cracked when the crown is seated.

The obvious follow up question that patients ask is why not cover the margin with porcelain in the first place? I usually choose not do this with my posterior restorations since I find that when my crown margins are covered with porcelain it can cause the crowns to fit too tightly since porcelain shrinks when it is fired and this can put pressure on thin and flexible metal that supports it at the margins. When trying in crowns with no metal collars it can sometimes cause the porcelain to 'pop' off and leave the porcelain with a chip.
If the technician chose to make the metal at the margin thicker than this wouldn't be as much of a problem, but the resulting crown would have a noticeable 'ledge' at the margin and could become a plaque trap.

I usually specify that my dental lab use a thin 'hairline metal collar' on the buccal of all my porcelain fused to metal crowns. This tiny metal margin is not usually noticeable since it has the thickness of one or two hairs and is subgingival. If a patients gums eventually recede it may become visible.

If a patient doesn't desire to have a grey line, they may consider a different type of crown (Lava, Emax, Procera, Empress) but often these crowns can be less strong or be more cement dependent than a porcelain fused to metal crown fabricated on a tooth with a bevel preparation. Although not all dentists probably will agree with me, I find that cement dependent restorations have less initial retention and tend to be more dependent on their cements to keep them attached to their teeth. If a crown is 'cement dependent' it has one more thing that may fail and often the cement is the weakest component of the restoration. It can separate either the tooth or the restoration. I have found that this can make some of these crowns more likely to become loose or come out down the road. Metal free esthetic restorations tend to have larger gaps between them and the tooth and are more likely to fail in the long run.

The pyramids were built without any cement and they have lasted for thousands of years. In my opinion, The cements used to lute pfms to teeth tend to seal up the tiny gaps between the metal and the tooth and do a better job in resisting the forces that might tend to dislodge a crown.