One of the most beautiful attributes of a pearl is the way it reflects light from both its surface and its many layers within. “Luster” describes a pearl's light-reflecting qualities and its deep-seated glow. The highest-quality pearls have many layers of mineral deposits, giving then the highest luster. While a low-luster pearl can look milky, a high-luster pearl captivates your gaze with iridescent, mirror-like reflections.
When making a pearl, an oyster or other mollusk lays down thin layers of unique mineral combinations that create its iridescent qualities. You may know this as mother of pearl—the same mineral layer you sometimes see lining the shell of a mollusk. In a pearl, it's called “nacre”.
Pearl Surface Perfection
As products of nature, pearls can have subtle variances and marks in their texture. These can arise from tiny particles enter the shell of a mollusk while the pearl is forming. The smoother the surface is, the higher the grade and more valuable the pearl.
Pearl shapes vary, with a perfectly round pearl being most rare and valuable. Shape is, in part, characteristic of the type of mollusk that produces the pearl, and it's also influenced by the variety of nature. Some pearls are disc-shaped, curved, or uniquely formed in a variety of ways. Pearls of highly unique shape are called “baroque”.
While we often think of pearls as white, there are many colors of pearls, depending on the species of mollusk that produces them. For example, a black-lipped pearl oyster produces black pearls. The color hues of pearls can span from golds and creams to pinks, greys, greens, and blues. For any color pearl, a sign of quality is even distribution of color throughout the pearl.
Pearl size is measured in millimeters (mm). Classic Japanese pearls made by small pearl oysters (akoya pearls) rarely exceed 8 or 10 mm. Pearls produced by oysters in the South Seas are larger, typically 8 to 18 mm. The largest pearls of any variety tend to be the most expensive.