Dating method
Optically stimulated luminescence
A way to estimate how long mineral grains have been in the dark. It dates a burial or a last heating, not automatically the day a picture was carved.
What is measured
Sunlight or strong heat empties a store of trapped energy in grains of quartz or feldspar. In the dark, that store fills again at a roughly knowable rate. In the laboratory a light releases what has accumulated, and the brightness is read as time spent buried or unheated.
The clock starts when the grains last saw the sun, or last passed through a fire hot enough to empty them. A stone carved in the open and never buried has not started that clock.
Why a single result is easy to overread
If the grains were only partly exposed to daylight before burial, the clock starts with time already on it, and the age comes out too old. Mixed layers, a wet sediment, and an uncertain dose from the surrounding soil all widen the range.
A date on the sand against a wall is a date for that sand. It becomes a date for the wall only if the sand really belongs to the building, not to a later wind or a later pit.
What would make it firmer
Several samples from the same event, agreeing with each other, and agreeing with radiocarbon or dendrochronology where those can see the same moment. One glowing number, from grains that may have been partly lit, stays a suggestion.