What Is Nacre?
You have heard this term used as a level of quality in a pearl. But, why?
Scientifically speaking, nacre is the interior layer of a mollusk shell and the substance created by the living body of the mollusk to create protective layers around an intruder into the body of the mollusk. Nacre is a combination of layers of aragonite platelets and a small amount of organic material to bind them. These layers are arranged in an overlapping brick-like structure.
Light is diffused by these nacre layers which causes the color of the pearls, as well. The colors are caused by pigments inside the nacre layers which are usually the same color as the nacreous lining of the inside lining of the mollusk. Hence, a gold lipped oyster would produce a golden pearl and a black lipped oyster would produce the levels of grey to black of a Tahitian pearl. Other trace minerals give added color depth, such as rose, peacock and pistachio overtones to white, golden, and black pearls.
Light diffusion through the pearl nacre layers also causes iridescence or "orient" of a pearl. This is that shimmering light and overtone that is so praised in a high quality pearl. Imagine the light entering a pearl and bouncing off these brick-like layers of aragonite to the center and back out the pearl at a different obtuse angle. This is the essence of the prismatic array of colors. High quality pearls with a tight arrangement of aragonite layers are not opaque. They have a magical, but entirely scientifically explainable, quality.
Now, you understand why you become hypnotized by looking into a magnificent quality pearl.