I'm going to partly copy my response from our other ongoing mercury topic

So within Mesoamerica I only know of two contexts in which mercury was used. The first is the Maya region. Several burials (Austin 1994) and ritual offerings there had been found with vessels containing liquid mercury. Within Maya culture at the time, and within Mesoamerica, "mirrors" were highly prized. These "mirrors" consisted of highly polished stones and were coveted for their reflective capability. They were most often used as decoration for costumes. Now, since so little mercury has been found and it is highly reflective, that is where the interpretation that it may have been a mirror comes from.

As for Teotihuacan, it could be simply that it was a neat substance and the people who buried these individuals decided to use it as a grave offering. It could also be that they were doing a similar practice to the Maya and burying a "mirror" near an elite burial. It is important because no elites have been found at Teotihuacan and no structures that could adequately be described as a palace have been identified. Or at least no burials in which we would consider the person to be elite. This makes their political system rather mysterious when coupled with the lack of portraits of rulers. Unlike the Maya region where the ruler glorified himself and his actions, Teotihuacan has repetitive murals depicting religious or semi-religious scenes. The people within the scenes are decorated in a costume and have no defining characteristics of individuality. This has lead some researchers to believe that Teotihuacan was not ruled by a single ruler, but perhaps by a group of rulers (Headrick 2010). Such a system is not unknown elsewhere in the rest of Mesoamerica. The Teuchitlan culture that I study, for example, is believed to be a corporate based society.

We won't know for certain until something gets published in a journal.

Austin, Patricia A.

1994 Mercury and the Ancient Maya. Unpublished Master’s thesis, Department of Anthropology, Trent University, Peterborough, ON.

Headrick, Annabeth

2010 The Teotihuacan Trinity: The Sociopolitical Structure of an Ancient Mesoamerican City. The University of Texas Press, Austin.