Some obsidians hydrate faster than others, even within the exact same depositional environment.
Making stone tools out of obsidian breaks the rind and starts the obsidian clock counting.
Eerkens JW, Vaughn KJ, Carpenter TR, Conlee CA, Linares Grados M, and Schreiber K. Obsidian hydration dating on the South Coast of Peru.
Obsidian hydration dating (OHD) is a geochemical method of determining age in either absolute or relative terms of an artifact made of obsidian.
If you can determine the rate at which water diffuses into the glass for that particular chunk of obsidian (that's the tricky part), you can use OHD to determine the absolute age of objects.
The relationship is disarmingly simple: Age = DX2, where Age is in years, D is a constant and X is the hydration rind thickness in microns.
C dating, thermoluminescence is related to radioactive decay.