When a tooth is heavily decayed it is more likely to fracture and lose considerable tooth substance. The greater risk is that a fracture line may go below the margin of the gum which makes restoration more difficult or near impossible.
Crowns may be made from several different materials, porcelain being the most common type.
Some porcelain crowns have metal inside which gives strength and support to the porcelain. This acts as a barrier for light and gives the crown a dull colour. On posterior teeth this won't be noticed but in the front of the mouth it can occasionally be a problem as the tooth looks duller than the natural teeth. Therefore all ceramic crown have gained wide importance over the last few years. With the advent of metal free zirconia crowns, there has been a revolution in esthetics as well as functional ability of the patient.