The eye was ridiculously hard to make.

I never expected that the item that would have the most complex shading process would be an eye. In hindsight this makes sense but going into it I did not expect it.

The eye is composed of two models and the shading on the outer one needs to be partially transparent which is done in shading by separating which parts are shaded how. 

So in short if if you learn how to texture the eye you can texture anything.

The progress of making of the eye pictures bellow.

Eye start to finish