This is an attribute you should take seriously into consideration when purchasing a pearl. And, of course, the higher the luster of the pearl, the more valuable it becomes. Thinly covered cultured pearls are usually dull and lackluster, since this effect is given by the many millions of thin layers of nacre. A pearl without luster is as beautiful as a polished bead of clam shell.
But to explain these effects we need to understand how a layer of nacre works. Nacre layers are very thin (500 nm or 0.5 µm) and translucent but also reflective. When light hits the surface of a pearl some of it will be bounced off the surface of the pearl (like in a mirror) but some of the light will also go through this layer and into the next one (just like like glass). The process then repeats itself millions of times, over and over. So each layer of nacre is bouncing light and deviating it, thus creating the effect of a soft, silky glow. This is deep luster.
Surface luster happens when the outer coatings of the pearl are very thin and the crystals are well organized, so they are able to reflect light. You can measure superficial luster by comparing it to the way a mirror reflects objects: the sharper your image on the pearl's surface, the more lustrous it is. This effect can be increased with polishing, but it can be affected by UV light, heat, and chemical substances, since pearls are delicate creations. And, as you can deduct from the picture above, a thinner pearl (left) has fewer coatings of nacre than a thickly coated pearl, so the effect of "deep luster" will not appear on thinly covered pearls. The thickly covered pearl has many more layers on which the light can play and bounce.
It is a very normal procedure for pearl farmers to send their pearls to be polished. Polishing adds more superficial luster to the pearl (but it cannot give you the much more desired "deep luster" effect) and this also takes away some outer coatings of nacre. Good quality pearls do not need polishing and are -as a matter of fact- deeply affected by this process (they can actually loose some of their superior natural luster). The Cortez Cultured Pearl is NEVER polished and will always remain 100% untreated.