When they first enter the chamber with the giant head, Charlie walks to the back of the chamber and looks at the scary Geigeresque mural. Enroute, he passes a table with a green translucent looking sundial thingy. What was that?
The DVS track answers nothing; it only describes what the question described. The three script drafts I checked, Alien: Engineers, Paradise, and Prometheus made no reference to this emerald-like stone. It seems answerable only by speculation at this point unless someone has a copy of the "shooting script" that explains it. I have not found a draft with any answers. I have rewatched the scene in context several times and scoured the net and can only find speculation. No canon or interview explanations. At this point either only the filmmakers know, or they left it as a possible toy to play with later.
Incidentally, the murals are not Gigeresque, they are simply Giger. He made them for the film, and according to this article they even used some of his old art from the unmade Alejandro Jodorowsky adaptation of Dune.
There is no definitive answer yet. Maybe in the sequel(s).
Prometheus is one of a small percentage of movies that feature a DVS (Descriptive Video Service) track, in which the action on the screen is described by a narrator for the visually impaired. The DVS description of the scene:
As Holloway steps around a large pedestal his flashlight beam shines through a huge emerald-like stone jutting from its center.
Other than that there is nothing but the movie itself to guide us. Nothing like that pedestal was ever used for anything in the film, so I think this is another piece of unexplained background among many.