Luster is the quality and quantity of light reflecting from the pearl's surface, or "brightness" of the pearl. Very high luster is a sign of high quality nacre. You should be able to see your own reflection clearly on the surface of a pearl. The finer, thicker and more uniform the nacre is, the more lustrous and blemish-free the pearl forms, and therefore it is more valuable and desirable.
Color is determined by the "mother of pearl", or lining, inside the oyster shell. Pearl color includes three components: the body color or the primary color, the overtone color (a tint that appears super-imposed on the body color) and a rare component called "Orient" (an iridescent of rainbow of colors that generally is found on baroque pearls or those with irregular surface). The most valuable in the light body group are pink, rose and white. In the dark body group, like Tahitian pearls, is black with a green overtone, known as "Pistachio".
Traditionally, pearls have been round, but pearls come in a variety of shapes and suit all fashion styles. Most recently, we have seen an increased interest in shapes other than round: "drop or teardrop" shape lend themselves to earrings and pendants, while baroque or irregular shapes can offer opportunities to create interesting design. Three things determine the size of a pearl: the size of the nucleus, the size of the oyster and the thickness of the nacre. Size is an important value factor.