Obsidian hydration dating is a method that allows for the 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).