Most eyeballs use 3 materials: One for the sclera, one for the Iris and one for the transparent shiny exterior layer. The Robot and reptilian eyeballs use their own materials. 

Materials

The materials are modular, and are defined per object instead of per data, so you can have different materials set to instanced/linked copies of the same object. To make everything easier, there are 6 node groups available, one is an internal node and the rest of them create the color and bump height data to connect into the shader's inputs.

Node groups:

All the materials make use of at least one of these node groups, so the node tree looks simple and clean. You can enter these node groups and make your own adjustments but I don't advise it unless you already have some experience with the Blender node system.

dEYE.Transparent.Layer.default

This is the exterior surface material, it controls the glossiness of the eyeball, and the hologram emission pattern in robot and android eyes.

dEYE.Sclera.Material.default

This controls the veins and red gradation in most of the eyeballs, except in the robot and reptilian ones.

dEYE.Iris.Material.Default

A mix of procedural textures and color inputs to create the radial pattern of the iris.

dEYE.Material.robot

Metallic material with 3 color inputs for the solid parts, and 1 for the emission light.

dEYE.Material.creature

A special group designed to work only with the reptilian eye.

RadialSilhouette

A metanode group, really. It works inside some of the other node groups creating black and white radial patterns.

How to: Append any eye into a Scene:

• Create or open a .blend file
• Press Shift F1 or File -> Append
• Find this .blend and select it• Go into Object
• All the eyeballs are named like this [Type].eye.[Color]
• Select the desired object
• Press "Link/Append from Library"