Tissue Nucleated Pearl

A tissue-nucleated pearl is a cultured pearl grown without a bead at its centre. In the more common bead-nucleated method a round shell bead is implanted with a scrap of mantle tissue, and the mollusc coats the bead in nacre; in tissue nucleation only a small piece of mantle tissue is inserted, and the pearl that forms is nacre nearly all the way through. This is the standard technique for freshwater cultured pearls, in which a single mussel may be seeded many times, and it produces pearls of solid nacre but usually irregular, non-round shape.

Like all pearls it is an organic gem, aragonite platelets bound by the protein conchiolin, its glow the interference of light in those thin layers, and it is soft, hardness about 2.5, and dissolves in acid. The distinction matters for value and identification but not for symbolism: in tradition all pearls share the associations of the Moon, in Indian jyotisha the pearl is the primary lunar navaratna gem, and of purity, and the culturing methods that make them affordable are a twentieth-century development.

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.