What is this shining in the skull?

Solution 1:

In all the instances I've seen of this set piece, this item is marked as a "Mysterious Socket" (and it's also not glowing to begin with, if I remember correctly). When you kill the champion level enemy near this area, a "Mysterious Gem" drops. When you place the gem into the socket, the eye begins to glow and 2 medium-sized chests appear from under the sand. Each chest contains some gold, random loot, and one "Basilisk Eye" gem (either a Yellow, Black, or Green variant). These gems have unique weapon effects, and can be particularly useful for low-level characters when socketed into armor.

Judging from your screenshot, it looks like you may have already socketed the gem and received the loot.