The gray or black indicates that there is metal underneath the white porcelain. The gray or black line you see around the crown was probably not there when the crown was placed. Over time, chronic inflammation of the gum tissue causes it to bleed easily and perhaps recede from the metal, exposing it. No longer is it necessary to have metal in crowns or that unattractive dark line. All-porcelain restorations are just as strong as metal supported crowns or bridges, and will last as long.