Japanese Akoya Pearls Luster
Nacre and luster are the most important quality measures. Nacre quality is determined by analyzing layer thickness and layer density. Both of these factors help us to determine luster. Since the average consumer is not able to determine nacre quality, the most import factor for the consumer to understand is luster, which can be judged easily by the naked eye.
Pearls with high luster sharply reflect images around them, almost like a mirror. They always display the characteristic depth, or three-dimensional glow, and a subtle display of different surface colors (orient) that is so prized among high-quality pearls. Pearls with low luster often look dull, chalky and lifeless.
Luster, in more scientific terms, is the reflection and refraction of light as it passes through the layers of aragonite platelet crystals that form a cultured pearl’s nacre. The intensity of luster is determined not only by the amount of these crystals but also by the specific geometric pattern and regularity in which they are secreted on the nucleus by the mollusk.
Many factors affect nacre secretion, determining the intensity of luster. The health of the oyster, the water temperature of the host environment and the level of nutrients available to the oyster all play a vital role in creating a pearl with superior luster.
Though the intensity of a pearl's luster can be an indicator of nacre thickness, and thus determine the durability of a pearl, this correlation is not always true. Some cultured pearls have very thick nacre combined with poor density resulting in mediocre luster. This goes back to the formation patterns of aragonite platelets. Platelets in a very loose formation cannot refract and reflect light as well as very tightly packed platelets (nacre density). The loose formation phenomenon occurs more often in pearls grown in warm water.
Chinese Akoya pearls are cultivated in warm waters throughout the entire nacre growing period. This promotes quicker/more nacre coating, but the coating is often what many experts call “soft”, non-durable and typically lacking good luster. The waters in which the Japanese oysters are farmed are much less polluted and subject to broader seasonal changes in water temperature. Cold water slows down the nacre secretion process, reducing the size of aragonite platelets and producing tighter, more densely packed nacre layers. Japanese producers conduct their harvest only once a year in the freezing winter weather to ensure that the cold-water pearl coating is at its peak. Temperature combined with minimal pollution gives the fine quality Japanese Akoya pearls their “mirror like” luster.