A cultured pearl is a real pearl by every physical test, and establishing that took a court case.
There are two methods and the difference between them is the whole of this entry. In bead nucleation, a sphere cut from freshwater mussel shell is implanted in the gonad of a saltwater oyster along with a small piece of mantle tissue from a donor animal. The tissue forms a sac that deposits nacre over the bead. The result has a large foreign core and a nacre layer that may be under a millimetre thick, and it can be identified by X-ray.
In tissue nucleation, only the graft of mantle tissue is implanted, with no bead. The sac forms and deposits nacre around itself, so the pearl is nacre nearly all the way through. This is the standard method for freshwater cultured pearls, which is why a freshwater cultured pearl is often more solidly nacreous than a saltwater one costing considerably more, and why the shapes are less regular.
The technique was developed in Japan in the early twentieth century, most publicly by Mikimoto Kokichi, and the natural pearl trade responded by arguing that the product was an imitation. The matter reached a Paris court in the 1920s, and the finding was that a cultured pearl is a pearl: the nacre is deposited by the animal and is chemically and structurally identical to that of a natural one. What is different is the cause of the irritation.
Disclosure, rather than authenticity, has been the issue in the trade ever since.