I have some wild and baseless speculation on that point:
Looking closely at the cap I just posted above, it seems to me that the body of the idol has a fine, hammered-metal-like texture, whereas the face appears almost completely smooth. If that's correct, I'm guessing the hammered-metal effect was deliberately sculpted into the idol, to make it seem more ancient and hand-made. I'm also guessing that the apparent fine grain effect on the face and head may have come from a dusting of some matte spray and / or dust, done during shooting for aesthetic reasons. All speculation on my part.
Also interesting is the OfficialPix image Franz Bolo posted on 5-30-2008. That seems to show an idol with solid gold eyes and black (painted?) pupils, which jives with what I think I can see in some of the 'action' scenes where a 'stunt' idol might have been used.
-Mike J.