Obsidian is a naturally occurring glass, usually black and opaque. Ancient tradition among many Mesoamerican cultures was the practice of divination using the surface of a bowl of water as a mirror. At the time of the Spanish conquest, this form of divination was still practiced among the Maya, Aztecs, and Purépecha. In Mesoamerican art, mirrors are frequently associated with pools of liquid; this liquid was likely to have been water. Check out the article from Harvard University: The Obsidian Mirror.