What environmental factor can significantly impact pearl luster?

The correct answer is water temperature because it plays a significant role in the health and growth of the mollusks that produce pearls. Warmer temperatures can accelerate the metabolic processes in these organisms, leading to faster nacre deposition. This nacre, or mother-of-pearl, contributes directly to the luster of the pearl.

When water temperatures are optimal, pearls are typically produced with a smoother, more reflective surface, enhancing their overall luster. Conversely, extreme temperature fluctuations can stress the mollusks, potentially leading to poorer nacre quality, which affects the final appearance of the pearl.

While water salinity, pollution, and acidity can also affect pearl formation and quality, they do not have as direct or pronounced an impact on luster specifically as water temperature does. Water salinity affects the overall health of the environment, pollution can lead to lower quality nacre or damage to the mollusks, and acidity levels can influence the biological processes in the water but are secondary factors in the context of luster compared to the direct influence of water temperature.