A complex refractive index is defined as $n = n_0 + \kappa$ where $n_0$ is the "standard" refractive index, and $\kappa$ is the optical extinction coefficient. The optical extinction coefficient "indicates the amount of attenuation when the electromagnetic wave propagates through the material". The larger the value, the more light is absorbed by the material.
The fresnel equations tell us how much light is reflected, vs how much is transmitted into a material. Because metals have a high $\kappa$ value, this means that they absorb light really well and so light cannot propagate through the metal. Therefore, metals cannot be transparent and any EM waves that enter them are absorbed (I understand they can be transparent to higher frequencies, as the refractive indices are dependent on frequency, but for right now I'm focusing just on ~visible light).
Naively, this would lead me to believe that if a material doesn't have a high $\kappa$ value, then it must allow light to propagate through it reasonably well. But this sounds like it means the material should be transparent, which obviously isn't always the case. Many compounds do not have a complex refractive index (or at least, have a very tiny imaginary component), and yet are completely opaque.
From some reading, it seems like what's happening is that the light is scattered in the material somehow. But shouldn't the light still eventually escape? And why would the material cause light to scatter to begin with? I've seen it suggested that imperfections in the material, such as the grain boundary between two grains of the material, form an interface which cause fresnel reflection/refraction to occur again. This seems like it could be reasonable but it feels incomplete, as it'd suggest a perfect crystal would be transparent then. And it still wouldn't explain how the light ends up getting absorbed by the material anyways.