Type SpecularCoefficient defines the reflection of ambient light on shape surfaces. If value = 0, the light is completely absorbed. Often, 0.7 is a reasonable value. It might be that from some viewing directions, a body is no longer visible, if the SpecularCoefficient value is too high. In the following image, the different values of SpecularCoefficient are shown for a cylinder:
type SpecularCoefficient = Modelica.Icons.TypeReal(min = 0);