FeCr Oxides
FeCo Oxides
MnFeCoNi oxides
Pr-doped zircon
Fe-doped zircon
CoAl spinels
From http://en.wikipedia.org/wiki/Refractive_index
"Definition: The refractive index n of a medium is defined
as the ratio of the phase velocity c of a wave phenomenon
such as light or sound in a reference medium to the phase
velocity vp in the medium itself ... The phase velocity is
defined as the rate at which the crests of the waveform
propagate; that is, the rate at which the phase of the
waveform is moving"
Does an opaque material have a refractive index?
Refractive index is wavelength-dependent, and "opaque" materials
still have measurable (but high, > ~2 - 3) RI.
You might Google "Mie scattering". The Mie equations are dependent
on complex refractive indices of both medium and particle.
David