Natural Pearls, formed without human intervention are a result of mollusk's natural defenses as they secrete protective smooth shell material (nacre) around irritants in order to isolate them within the mollusk's own inner-shell material. 

Cultivated pearls are formed much the same way, although the "irritant" (generally a spherical shell bead and or a piece of mantle tissue from a host) is implated in the mollusk by man.  Under cultivation, the optimal conditions for pearl formation are maintained and yet, even then, the yield of perfect pearls of superior quality is still minimal.  Therefore, while affordable, the best pearls are still rare and require a significant investment.  Because the conditions for pearl formation are so closely controlled, cultured pearls can be much more beautiful than most natural pearls.