Eye perhaps used for the coffin of an Apis or Buchis bull. The large size (the piece is 105mm across) makes it seem unlikely that this was used to represent a human eye.
The ‘white’ of the eye is made from limestone and the pupil, as with many other eyes of statues and coffin, is made from obsidian (volcanic glass). The pupil itself is loose. If it is removed and turned over, this is what the back looks like:
Note the ridges. This shows that the piece was knapped, like flint is knapped. This section was struck off a larger piece of stone. You can see the ripples from the impact made by the blow.
Obsidian is not known from Egypt itself. It may have been imported from Ethiopia or the Aegean.