Blue Spodumene is sensitive to the UV spectrum. The material is more reactive to the shortwave spectrum (UVB) centered around 254nm versus the longwave (UVA) between 320 - 400 nm. Since natural sunlight contains UV, it can damage the optical properties at a slower rate.
I may not be able to articulate my understanding of the phenomena with true brevity… but here we go:
There are two related phenomena:
Photochromism - “The permanent change in optical properties to a crystal lattice, at the molecular level, due to the effect of ionization from electromagnetic or higher energy radiation sources.”
Tenebrescence - “The temporary or reversable change in optical properties to a crystal lattice, at the molecular level, due to the effect of ionization from electromagnetic or higher energy radiation sources.”
The stage: Some crystal lattice structures are inherently weaker than others based on the type of crystal system and the associated internal bonding points at the atomic level. This is evident with the durability and hardness a gemstone displays.
The interaction: As light passes by an atom in a crystal lattice, its vibrational energy can transfer energy into the valence electron shells. The natural resonance for a particular element or molecule will have a fundamental frequency and several harmonic orders. The magnitude of the interaction of the light and crystal lattice is observable as emission or absorption lines in a spectroscopic scan or image. Impurities or trace elements are commonly present providing the distinctive colors we observe visually and identified spectroscopically. The crystal lattice can contribute or generate the natural hues we see visually based on the optical properties.
The effect: The energy transfer from the light to the crystal’s atomic structure can cause a bond point to break or change its alignment with an adjacent atom or molecule chain. The natural resonance frequency will change. When this occurs, the visible spectrum we observe will change.
Interesting fact: It is understood that natural blue spodumene develops its visible color hue and saturation while in close proximity to radioactive minerals in the local matrix during its crystal growth. There has been some experiments attempting to recover the natural color hues in spodumene using irradiation treatments. The effects are short-lived and not permanent.
The following link has some excellent literature that dives deeper into the fundamentals, that I poorly attempted to dialog here. I recommend reading chapters 5, 10 and 11; but the entire text book is worth studying and retaining as a resource.