Many "old technology" restorations are designed with a metal lining covered with porcelain (porcelain fused to metal). They appear "flat" because the metal does not let light pass through like a natural tooth does. There is often a dark line next to the gum line that is undesirable, and often the metal irritates the adjacent gum tissue in the form of an allergic reaction.
Always inquire! Dentists will say that it is porcelain. And it is, on the outside.
Dr. Adler provides 100% all-porcelain restorations. When properly seated, they are as strong as or stronger than their metal predecessors. And the appearance is that of a natural tooth, allowing light to pass through (referred to as translucency).