Natural pearls are found, not grown. Only one in every 10,000 oysters may grow a jewelry grade pearl. Natural pearls have not been harvested commercially for 100 years, so the vast majority of pearls on the market today used for fine jewelry are cultured. This is includes every pearl company (including Mikimoto and Tiffany) in existence today.
A natural pearl begins its creation as a foreign object. A piece of sand gets lodged inside an oyster's inner body where it cannot be expelled. The oyster goes into defensive action and secrets a substance called nacre around the irritant to protect itself. For how ever long the irritant remains within the body the oyster will continue to secrete nacre around the irritant, adding layer upon layer.
The number of natural pearls is relatively small. Without cultured pearls, only the extreme wealthy would have the chance to own the treasured gems. Man-made pearls are produced in factories without the help of oysters but organic and inorganic compounds and chemicals. A popular technique includes taking a glass bead and dipping it in a fish-scale and lacquer substance. You can tell by pearl shine on the surface whether it is man-mad or cultured because artificial pearls shine on the surface like nail polish. Cultured pearls have an authentic looking glow that comes from the inside
Cultured pearls are formed within the oysters in almost the identical manner as their natural counterparts with the only difference is being that man implants the irritant - a small piece of polished shell and then lets nature and the oyster, create the work of beauty.
It takes more than 800,000 oyster hours to grow cultured pearls considered of "high quality."
A pearl farmer has to sort through 10,000 pearls in order to gather 47 matched pearls that will make a high-quality necklace. In an assembled strand all pearls must match in luster, shape, color, and size.
Today the vast majority of pearls bought are cultured pearls. This is because natural occurring pearls are so rare that they are seen today mostly in estate jewelry collections.
All Japanese cultured pearls have the nuclei shell beads come from freshwater mussels that are from the United States.
Pearls do not have an overall grading system because each pearl is different, some are so unique that they would not be able to be compared, so to have a grading system would mean there would have to be hundreds of quality parameters. Thus, there is no grading system for pearls.