Wired News: Old Stones Reveal Their Age: "The new technique, called quartz hydration, takes advantage of the natural properties of quartz, a mineral found in many rocks. Whenever a rock containing quartz is cut or polished, perhaps for a statue or ax head, the quartz at the surface is left exposed. Over time, water diffuses into the quartz, forming a layer. By measuring the layer, Ericson and his team members found that they could determine how long ago the rock was cut.
The technique can be used to date artifacts that were created between 100 and about 100,000 years ago, Ericson said. "