Which element is generally used to enhance the luster of pearls during culturing?

The enhancement of luster in cultured pearls primarily involves the use of calcium carbonate. Pearls are formed when mollusks secrete nacre, which is primarily made up of aragonite and calcite, forms of calcium carbonate. During the culturing process, the quality and thickness of the nacre that surrounds the bead nucleus are essential in determining the pearl's luster.

When cultured pearls are treated or induced to secrete additional layers of nacre, it results in a deeper and more lustrous appearance, directly relating to the calcium carbonate content. This is because the nacre’s composition and structure have a direct impact on how light interacts with the pearl, ultimately enhancing its luster.

Other elements such as sodium chloride, magnesium sulfate, and silica gel do not play a significant role in enhancing the intrinsic beauty of pearls through the nacre-building process. Hence, the use of calcium carbonate is critical in the cultivation and luster enhancement of pearls.