The most beautiful crown for a tooth is, without question, all-porcelain or all-ceramic. With porcelain fused to metal crowns, there has to be an opaque layer put over the metal to block out its color. This makes it impossible to have a translucent restoration that mimics the translucency of natural teeth. Only with pure porcelain or pure ceramic can you have such translucency.

The reason all dentists don't use all-porcelain crowns for front teeth is that the technique for placing them is very demanding and isn't generally taught in dental schools. They are translucent, and their color is influenced by the color of the underlying teeth. General dentists aren't usually very skilled at color manipulation in these situations. 

CHOICES IN ALL-PORCELAIN AND ALL-CERAMIC CROWNS

There are various types of all-porcelain or all-ceramic crowns. Let's explain the differences between some of them:

Feldspathic porcelain - is the standard, traditional porcelain that is used for crowns. Many cosmetic dentists feel that this is the most beautiful porcelain.

The Empress crown - Empress is strictly speaking not a porcelain, but is more like a glass. It can be called a ceramic material. The Empress material is cast rather than baked as a feldspathic porcelain crown is. The fit of Empress is more precise than the baked feldspathic porcelain. However, the color in Empress is mostly baked on the outside. Empress can be very beautiful. For appearance's sake, some expert cosmetic dentists prefer the feldspathic porcelain, and some prefer the Empress.

The Procera crown - Procera is a milled ceramic on the inside with a more traditional porcelain baked onto the outside. The advantage of Procera is its exceptional strength. However, the milled ceramic core is opaque white, so many cosmetic dentists feel that it isn't as natural-looking as the more translucent materials. An advantage of Procera is that it doesn't have to be bonded to the tooth but can be cemented with ordinary crown and bridge cement, a technique familiar to all dentists.

The Lava crown - Lava is similar to Procera, but the milled ceramic on the inside is a more translucent Zirconia, rather than an opaque white material. The Zirconia is shaded, and then the final esthetics of the crown are achieved in the baked-on outer layer. The Lava crown can also be cemented with traditional techniques. However, any crown cemented with a traditional crown and bridge cement is going to be susceptible to a compromise in the appearance if that cement line ever shows.

Zirconia crowns - if they are done right, can be translucent enough to look natural. And if they are bonded to the teeth, instead of being cemented with conventional dental cement, they won't show a black line at the gumline.

The Cerec crown - Cerec is are also milled from a block of very hard ceramic material. What's unique about Cerec is that the crown is milled by a computer in the dentist's office rather than in a separate dental laboratory. Thus, the dentist doesn't have to send out for it to be made—it can be made on the spot. So, no second appointment is required, and no wearing of a temporary crown between appointments. Cerec is milled from a block of ceramic that is a single color, so it is generally not considered esthetic enough for demanding cosmetic dentists. A few exceptional dentists who are artists, however, are able to custom stain Cerec for front teeth so that they are truly beautiful. Some even make Cerec veneers that can be placed the same day.
To be precise, Cerec is actually a technique and not a material. There are several companies that make ceramic materials for use in Cerec machines.

The InCeram crown - InCeram is made of a very dense and very tough aluminous porcelain. It also has excellent esthetics, but is more opaque than feldspathic porcelain. InCeram is also strong enough to be cemented with traditional dental cement.