Cultured pearls -
Cultured pearls are the response of the shell on a tissue implant. A tiny piece of mantle tissue of a donor shell is transplanted into a recipient shell. This graft will form a pearl sac and the tissue will precipitate calcium carbonate into this pocket.
There are a number of options for producing cultured pearls: use freshwater or seawater shells, transplant the graft into the mantle or into the gonad, add a spherical bead or do it non-beaded.
The large majority of saltwater cultured pearls are grown with beads, the trade name of the cultured pearls are Akoya, white or golden South sea, black Tahiti. The majority of beadless cultured pearls are mantle-grown in freshwater shells, tradename Chinese cultured pearls.
Cultured pearls (beadless or beaded) and imitation pearls can be distinguished from natural pearls by X-ray examination. Nucleated cultured pearls are often 'pre-formed' as they tend to follow the shape of the implanted shell bead nucleus.
Once the pre-formed beads are inserted into the oyster, it secretes a few layers of nacre around the outside surface of the implant before it is removed after six months or more.
When a cultured pearl with bead is X-rayed, it reveals a different structure to that of a natural pearl. A beaded cultured pearl shows a solid center with no concentric growth rings, whereas a natural pearl shows a series of concentric growth rings. A beadless cultured pearl (whether of freshwater or saltwater origin) may show growth rings, but also a complex central cavity, witness of the first precipitation of the young pearl sac.
Source: Wiki