If you make a metal thin enough it will become transparent; if you make a
ceramic thick enough it will become opaque.
It's a function of the wavelength of light and the thickness and which
wavelengths get absorbed or are transmitted.
Whether a material is opaque or transparent is determined by whether or not
a (visible light) photon will be absorbed, reflected or transmitted by the
material. The explination is quite lengthy but consider this example:
sillicon is opaque to visible light but it is transparent to infra red
light. Glass (sillicon dioxide) is transparent to visible light, but opaque
to unltra violet (ie: you won't get a sunburn if you stand infront of a
The conduction or outermost electrons in the metal are "free" or unbound
and you can sense that by the high electrical conductivity of the metals.
These "free" electrons will interact with the electromagnetic field of
light.... by absorbing it.... and the metal is opaque.
The conduction or outermost electrons in the ceramic are bonded to other
atoms. They are not "free" and won't ordinarily interact with visible
wavelengths of light (electromagnetic wave).
But another reason that ceramics are opaque is internal light scattering
from porosity and second phases (with differing indices of refraction)
and grain boundaries.
All of this scattering will usually make a nice white opaque ceramic.
Even if the small crystals of the ceramic are otherwise transparent.