Look into using a texture mask for subsurface color on a subsurface model. It’s an approximation that renders as a wholly opaque object and the texture is flat, so there’s no real translucency or volume, but you do get proper semi-translucent subsurface scattering/backlighting and reflections, which are the most important aspects to convey when rendering crystal.
But 3D volumetric textures? Um, that’s highly experimental, and not all GPUs can handle it. I know UE4 engineers are looking into volumetric effects for clouds and volumetric decals and such, but it might take some time before the proper tools come out to be able to literally design a 3D object with internal layers. In fact, there’s no standard yet for how such data should be handled. It’s not like an FBX file that works with everything from Maya to Blender and UE4, this kind of volumetric modeling is very new, and as far as I’m aware, doesn’t exist yet.