absolute or relative dating of an obsidian sample. The degree of hydration (i.e., water absorbed into the material) observed indicates how long it has been since the obsidian surface was exposed (i.e., through flaking). Obsidian hydration dating is at times, however, unreliable. The rate of hydration can vary tremendously depending on annual rainfall and humidity levels, among other factors, and how these have varied since the piece was first produced (or how they vary if the piece moved from one ecological zone to another). Due to the nature of the geological formation of obsidian, and the impact that each unique formation incidence has on the appearance and geochemical properties of each source, the material serves as an excellent medium by which long-distance trade can be studied. In performing trace-element or visual analyses, the origins of an artifact's material can be determined. It is clear that obsidian was a critical material in Precolumbian Mesoamerican economies; it is ubiquitous throughout the region, and found in the material record of all cultures and time periods. The low bulk of obsidian in transport, which therefore required less effort in trade, and the large quantity of useful items that could be produced from a small amount of material, greatly contributed to obsidian's widespread use. One example is the presence of Pachuca obsidian from central Mexico, where Mexico City is now, and ostensibly under the control of Teotihuacan, in the Maya area during the Early Classic. While the Maya had access to a number of local obsidian sources more readily available and (relatively) easily obtained, including El Chayal its main source, Pachuca obsidian remained an important trade good. The Olmec, from the Gulf coast likewise obtained its obsidian also from El Chayal in Guatemala (Andrews (1990: 13). It is unclear if trade for foreign obsidian contributed to the growth of Maya polities, or if it simply served as a mode for obtaining superior items or human labor. Generally, obsidian came into the Maya area via larger central places, such as Tikal, Uaxactun, and Palenque. Obsidian artifacts and tools were then redistributed to smaller and potentially dependent centers and communities. This is indicated by a lack of production debitage, including polyhedral cores, decortical flakes, and large percussion flakes, among rural occupations. Obsidian was generally transported, where applicable, along coastal trade routes. Of primary importance is the circum-peninsular trade route that linked the southeast Maya area to the