The thicker the nacre, the higher-quality they are. A thick nacre guarantees a pearl with good luster and color. However, you can't determine the thickness just by looking at them. You need to actually touch and feel the pearls in order to accurately determine their nacre thickness.
Nacre is a pearl's protective layer found on the surface of their shell (or mother-of-pearl). It acts like an armor that protects it from scratches and the environment. The thickness of the nacre is directly related to pearl quality, so if you can feel it with your fingers then that means they are high-quality pearls.
Use a needle, a toothpick or a pin to gently scrape the surface of your pearls. If you see no marks on the surface, then that means it's high-quality nacre and should have a good luster.