Which phenomenon is associated with light reflecting from the pavilion of a gemstone?

The phenomenon primarily associated with light reflecting from the pavilion of a gemstone is brilliance. Brilliance refers to the amount of white light that is reflected back to the observer's eye through the face of a cut gem. This reflection occurs when light enters the gemstone, is refracted, and reflects off the facets located on the pavilion (the lower portion of the gemstone). The arrangement and quality of these facets greatly influence how well light can be captured and reflected, contributing to the overall sparkle and visual appeal of the stone.

Extinction, which is the condition where certain parts of the gemstone do not reflect light due to misalignment or specific orientation of the facets relative to the light source, does not directly relate to the intended reflection of light from the pavilion that contributes to brilliance. Windowing refers to the appearance of clear openings or "windows" in a gemstone, which can detract from its beauty and brilliance. Color zoning describes the variations in color within a gemstone and also does not pertain to the reflection of light from the pavilion.

Thus, the relationship between the pavilion's facets and the reflection of light is integral to understanding brilliance, making it the correct choice in this context.