The thicker the nacre of a pearl and the better the quality, the more exquisite the pearl.

The thicker the nacre, the more lasting the pearl. Thin pearl equals short life. Simple. The farmers that produce the finest thickest nacre leave the mollusk to develop the nacre for the longest. Of course, this takes time and time equals money or value. 

Nacre quality determines how light travels through the layers. When the layers of nacre are the thickest and aligned correctly, the light will travel through these layers to create a prismatic effect. It's magical. 

The temperature and quality of the water can affect the development of nacre, as well. The South Sea and Western Australian waters are the warmest pearl producing waters, while the Japanese waters are the coldest. The warm waters produce the thickest nacre and the fastest growing nacre. South Sea Pearls usually have a longer cultivation period so they will have the thickest of nacres. Japanese pearls have a much shorter cultivation period, but the cold waters produce a sharper brighter luster.

There are ways to judge a pearl's nacre without cutting it open.

First, look for pearls that have a sharp bright luster. These will have a good thick luster. They will reflect nearby images. Avoid any pearls that look chalky or dull.

Second, look for orient, which is the iridescence on the surface of the pearls. It's the play of overtones of color that will entice you.

Third, you will of course, pass on any pearls that are peeling or are cracked. This is a vivid sign of thin nacre. You would be surprised what I have seen at trade shows that are being passed off as Tahitian or South Sea pearls. Look at the drill holes. That will be your first sign of a peeling or cracked pearl.

The more pearls you see, the more you will be able to judge the quality of the nacre. If you have a number of different strands of pearls, you can line them up and look for some of the characteristics that I have described. Good luck.