The difference between a natural pearl and a cultured pearl is that natural pearls are a product of nature and cultured pearls are a product of nature helped along by science. A nucleus is implanted into the mollusk to stimulate nacre production and over a few years nacre builds up layer by layer ultimately creating the pearl. The nacre is the key determiner in the pearls luster. Although the pearls are cultured, this doesn't mean they are all beautiful. In fact, many are not, especially for the larger pearls which are in the mollusk for a longer period of time and susceptible to more adverse conditions. This is why larger round cultured pearls are so round. Natural pearls and cultured pearls are produced in rivers, lakes, and bays by mollusks.