Editing Pearl (section)

===Cultured pearls===
{{Main|Cultured pearl}}
[[File:Pearls.jpg|thumb|left|Nuclei from [[Mikimoto Pearl Island]], [[Toba, Mie|Toba]], [[Japan]]]]

Cultured pearls are the response of the shell to a tissue implant. A tiny piece of mantle tissue (called a ''graft'') from a donor shell is transplanted into a recipient shell, causing a pearl sac to form into which the tissue precipitates calcium carbonate. There are a number of methods for producing cultured pearls: using freshwater or seawater shells, transplanting the graft into the mantle or into the gonad, and adding a spherical bead as a nucleus. Most saltwater cultured pearls are grown with beads. Trade names of cultured pearls are Akoya ({{lang|ja|阿古屋}}), white or golden South Sea, and black [[Tahitian pearl|Tahitian]]. Most beadless cultured pearls are mantle-grown in freshwater shells in China and are known as freshwater cultured pearls.

Cultured pearls can be distinguished from natural pearls by [[X-ray]] examination.<ref name=farn2013>{{cite book|first=Alexander E.|last=Farn|title=Pearls : Natural, Cultured and Imitation|place=Burlington|publisher=Elsevier Science|year=2013|isbn=9781483162737|pages=90–108}}</ref> Nucleated cultured pearls are often 'preformed' as they tend to follow the shape of the implanted shell bead nucleus. After a bead is inserted into the oyster, it secretes a few layers of nacre around the bead; the resulting cultured pearl can then be harvested in as few as twelve to eighteen months.

When a cultured pearl with a bead nucleus 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.<ref name=farn2013/>