Further study showed it's made of bitumen and discovered traces of gold and colour. In it's original condition, the eye was white with an iris and pupil.
The superb skills of manufacter used fine gold wire to represent the capillaries! Triangles were traced emerging from the iris. The effect must have been fascinating at a time of superstition and beliefs in seers, prophets and oracles.
Thanks for that link. It’s quite explanatory. The image of the reconstructed face is lovely.