Cultured pearls are the shell’s answer to a tissue implant. A small piece of mantle tissue (called a graft) from a donor shell is transplanted into a recipient shell, resulting in the formation of a bag of pearls in which the tissue precipitates calcium carbonate.
There are several methods of producing cultured pearls: using freshwater or sea shells, transplanting the graft onto the mantle or gonad, and adding a spherical pearl as a core. Most saltwater cultured pearls are cultured with beads. The trade names for cultured pearls are Akoya (阿古 屋), South Sea white or gold, and Tahitian black. Most of the beadless cultured pearls are cultured in freshwater shells in China and are known as freshwater cultured pearls.
Cultured pearls can be distinguished from natural pearls by X-ray examination.  Nucleated cultured pearls are often “preformed” as they tend to follow the shape of the implanted shell core. After a pearl has been inserted into the oyster, secrete a few layers of mother of pearl around the pearl; The resulting cultured pearl can be harvested in as little as twelve to eighteen months.
When a cultured pearl with a bead core is x-rayed, a different structure is revealed than a natural pearl (see diagram). A beaded cultured pearl displays a solid center with no concentric growth rings, while a natural pearl displays a series of concentric growth rings. A cultured pearl without grains (both sweet and salty in origin) may have growth rings, but also a complex central cavity, witnessing the first precipitation of the young sack of pearls. 
Main article: Imitation pearl
Some imitation pearls (also called shell pearls) are simply made of nacre, coral or shell, while others are made of glass and coated with a solution containing fish scales called essence of the East. Although faux pearls look great, they don’t have the same weight or smoothness as real pearls and their luster will also be noticeably softened.
A well-equipped gem laboratory can distinguish natural pearls from cultured pearls by using gemological X-ray equipment to examine the center of a pearl. With X-rays it is possible to see the growth rings of the pearl, where the layers of calcium carbonate are separated by thin layers of conchiolin. Differentiating natural pearls from beadless cultured pearls can be very difficult without the use of this X-ray technique.
Natural and cultured pearls can be distinguished from imitation pearls using a microscope. Another method of testing knockoffs is to rub two beads together. Faux pearls are completely smooth, but natural and cultured pearls are made up of nacre plates, which make them both look slightly grainy.