The esthetic appearance of natural teeth is produced because of the optical properties of both enamel and dentin. From cervical margin to incisal edge the appearance of a natural tooth changes due to the combination of enamel and dentin. When combined these two layers create the shade variety, translucency and opalescence we see in teeth.
Mimicking this effect with composite depends on understanding the light and color properties of natural teeth and the materials we are using. No one composite has the exact properties of enamel over dentin. Therefore, like nature we will layer composites of varying color and translucency. Hopefully the additive effect will copy the natural teeth surrounding the restoration.
Enamel allows approximately 70% light transmission, while dentin is closer to 50%. None of our composite materials have the same percentage light transmission. Composite materials are available as enamel shades, dentin shades, opaque shades and in some systems opalescent shades. Most enamel composites are less translucent than in reality. Conversely, most of the dentin composite materials are more translucent than natural dentin. Additively we can reproduce the light reflection and transmission of a tooth. Given the properties of composite, it is difficult to impossible to match the tooth with the application of a single shade.
In addition to the different light properties between enamel and dentin, the thickness of the two layers varies depending on the tooth anatomy. This variation in thickness creates what we call chroma gradient. In the cervical region of a natural tooth the enamel is .3-.5mm in thickness and becomes thicker until the incisal edge, which will have areas of no dentin. The interproximal contacts of anterior teeth tend to be enamel from facial to lingual as well, with no dentin in between.