Obsidian

Obsidian is just cool. It is volcanic glass and has been used throughout history and cross-culturally as devastating weapons and tools. It is one of the sharpest materials, though it is brittle, and has even been used as scalpel blades. It has been and still is used for scrying (to fortell the future using a crystal ball or other reflective object or surface) which is why it seemed is an apt material to emulate a pupil.