Oxidation isn't just gaining oxygen its usually defined as loss of electrons.
An oxidation is usually followed by an opposite reaction called reduction which is the gain in electrons.
Sometimes when light shines on certain minerals or oils, the UV rays emitted can actually transmit energy to certain molecules within the minerals because of the specific frequency of UV light.
When the energy is transmitted, the molecules gain enough activation energy to start reacting with each other.
Among that, there are several other things which may happen.
During oxidation reactions, elements can change oxidation states, that is, the number of electrons present in their ion for example Copper(I) has one electron missing from its ion, it usually appears brick red in colour. When oxidised, it becomes Copper(II) which now has 2 electrons missing from its orbital and its colour changes to blue.
This could explain how sometimes stones get dicoloured in sunlight.
In theory, it need not be just sunlight which triggers oxidation but it could simply be a UV lamp.
those are my thoughts anyway...
