"CALIBRATED RELATIVE". Obsidian Hydration Analysis - this technique involves the absorption of water on exposed surfaces of obsidian (glassy volcanic rock); when the local hydration rate is known, the thickness of the hyrdration layer, if accurately measured, can be used to provide and absolute date.* There is much local variation. An original known date to one piece of obsidian needs to be known in order to date other pieces. (This can be done when obsidian artifacts are found with other datable objects.) Tools were made of chipped obsidian. Works back mostly 10,000 years (ca.), but can go much earlier.
*definations from Archeology: Theories, Methods, and Practice by Colin Refrew and Paul Bahn. © 2000
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