We have tried to make this section both illustrative and entertaining. Here you will find information that can help you better understand and distinguish between different kinds of pearls. Of course, this information might span one large book on the subject (or several ones), but at least here you can get a quick compendium of information in plain English.
Natural pearls are an extremely difficult kind to come by nowadays. You have to slaughter thousands of pearl oysters in order to find just one Good quality natural pearl. In the best pearl beds inside the Gulf of California, -called "placer" in Spanish- the incidence of natural pearls was said to be in the range of 5 to 12%, meaning that for every 100 killed oysters you would only find some 5 to 12 pearls.
Yet, you would have to take another factor into consideration: of those pearls, only 30% would be of good quality. So, out of those possible 12 pearls, only 3.6 good pearls could be obtained. This intensive fishing effort had its adverse results on the viability of the natural pearl oyster populations throughout the World, most becoming exhausted beyond help.
Natural pearls are formed when the pearl oyster forms something called a pearl sac. This pearl sac is the result of external stimuli (a parasite -mostly polychaete worms- or even other form of mechanical damage) on the nacre (commonly called mother-of-pearl) producing cells of the oyster's mantle. This pearl sac engulfs the foreign body, and starts depositing thousands of very thin layers of nacre over it, encapsulating it to protect the oyster. After some years (3 or more), a Natural pearl of good size (4-12 mm) could be found inside an oyster.
In essence, a Natural pearl is identical to an onion in its inner structure: both are made of overlapping layers.
Natural pearls are -sometimes- worth around 10 times more than their cultured pearl equivalent.
Cultured pearls are formed by the joint efforts of Man and Mollusk, as opposed to a natural pearl. A cultured pearl is made of the same substance and formed in a manner similar to a natural pearl, but inside a cultured pearl you will have a mother-of-pearl bead, not a parasite. Cultured pearls are produced inside farm-reared pearl oysters (if done in a Sustainable manner).
A cultured pearl must not be confused with a fake or artificial pearl (like "Mallorca" or "Majorica" pearls). Imitation pearls are not real pearls but man-made products.
To grow a cultured pearl, you first need a Pearl Oyster. The oyster is induced to produce a pearl, taking advantage of the oyster's own natural pearl production scheme. This procedure is called grafting (but also known as the "pearl seeding operation") and its scientific basis (known as the "Pearl Sac Theory") was discovered at the beginning of the 20th Century by Australian researcher William Saville Kent, and his discovery was later put to work in a commercial manner by the Japanese.
Nowadays, cultured pearls are grown within many different species of pearly oysters and mussels, so it can -sometimes- get a little confusing for some people to distinguish among the many varieties of pearls available.