A dental crown is when the surface of the tooth is replaced with an artificial layer and is done when there has been an extensive cavity or trauma to the mouth. The replacement cover or crown usually takes two trips to the dentist; one to take an impression of the tooth to be covered and to prepare that tooth for the procedure and then a second appointment to actually fit the crown. Crowns can be composed of porcelain or Belle Glass (plastic and glass fibre mixture) depending where in the mouth the crown is to be placed.