The Stone Headdress is a key artefact found in the Tomb Raider Underworld level Xibalba. Lara needs it to use with the second Mayan Calendar. Without this important piece Lara will be unable to move the outer ring.
It is found in a temple at the end of the path leading straight away from the Mayan Ball Court. In order to enter the chamber where the headdress resides, Lara has to smash in the floor using the grapple to pull down a stone block.
Description in PDA
A stone carving of a skull wearing a ceremonial headdress. Most likely it is only part of a larger statue.