The four S's for determining pearl quality: shine, shape, size and surface.
Shine. Commonly called luster. Luster refers to the quality of the nacre as determined by the amount of light reflected off the gem's surface. While it is possible for pearls to have thick nacres and low lusters, high-luster pearls usually have thick nacre. High-luster pearls command a stiff premium over duller pearls that may have the same nacre thickness.
Shape. The rounder it is, the higher the value. Most pearl necklaces are off-round. "An oyster has no reason to make a round pearl," Asher says. Often, natural elements will imbed themselves in the forming pearl, which causes the mollusk to secret nacre around the new irritant. The result is an off-shaped -- or baroque -- pearl. While consumers might find them appealing, they command a fraction of the cost of round ones.
Size. Other qualities being equal, the larger the pearl, the more it will cost. Freshwater pearls generally range in size from 3 mm to 7 mm, Akoyas from 6 mm to 8.5 mm and South Sea and Tahitian pearls around 15 mm. To put that in perspective, 6 mm is about the size of a pea and is roughly the equivalent of a 1-carat diamond. Fifteen millimeters is the diameter of a quarter.
Surface. Like most gems, flaws will affect the value. But, because they are so fragile, pearls often have slight imperfections that don't necessarily reduce the value dramatically. But noticeable spots, pits or ring-like markings should not be present on high-end pearls. Pay particular attention to the drill-hole area and make sure there are no cracks or chipping. They will get worse over time.