These are the most common category. These are non-conductive materials, which don't have free electrons. The result is that dieletrics have low reflectivity, and have a reflectivity that is independent of light color. Because they don't interact with the light much, dieletrics tend to be transparent. The ambient, diffuse and specular colors tend to be the same.
Powdered dieletrics tend to look white because of the high surface area between the dielectric and the surrounding air. Because of this high surface area, they also tend to reflect diffusely.