I originally wanted to design a beholder-like creature, but ended up with this. Perhaps you can put it to good use.

Take note of the realistic flesh material used for the optic nerves. Nothing is textured, only materials; which are included.

The video below is a full 360 degree rotation. It was done in draft quality mental ray rendering, yours will be better if done in final quality mental ray (pictures are final quality).

The iris will react to light differently as angles move (think holographic) it is subtle, changes slightly from blue to blue-green in some areas. Depends on the lighting you use.