Pearl Nucleus Defined
A pearl nucleus is a bead or other object implanted into the the gonad of a marine mollusk or into an existing pearl sac in the gonad or mantle of a freshwater mussel around which nacre deposition occurs.
The Pearl Nucleus and Nucleation
Pearl farms vary greatly depending on the type of pearls being produced. But the one thing all pearl farms share is the process of creating the pearl nucleus, and nucleating the pearl. Every pearl produced commercially today, except Keishi pearls, are a result of nucleation, whether it be beaded or tissue nucleation.
The Units of Pearl Nuclei
The discovery of the attributes of the freshwater mussel shell in nucleus production is attributed to Mikimoto, who experimented with many materials. Due to this Japanese influence, nuclei are still sold today universally utilizing the ancient Japanese unit of length, the 'bu'. One bu is equivalent to 1/100th of a shaku. The metric value of 1 bu is 3.03mm, and the English value is .1193 inch.
Saltwater Pearl Nucleus
The nucleus used in all saltwater pearl farming today is most often a mother-of-pearl bead. This bead is composed of mussel shell that has been cut, rounded, and polished. A nucleus of high quality will be white, without calcium carbonate streaking (striation or banding), that may show through the pearl nacre.
How the Nucleus Creates a Pearl
The first step in nucleating saltwater mollusks is making a small incision to the gonad. The mother of pearl nucleus is inserted into this incision, which is then followed with a very small piece of mantle tissue from a donor mollusk. The mantle tissue is placed between the mother of pearl bead and the gonad, with the side containing epithelial cells facing the nucleus. These epithelial cells are the catalyst of the pearl sac. The pearl sac then grows around the nucleus and begins to deposit nacre. This nacre layering is what creates the beauty of the pearl.
How Many Times Can a Mollusk Be Nucleated?
Saltwater mollusks will only produce 1-2 pearls per typical nucleation. Akoya can be nucleated with up to 5 beads, but the use of only 2 is most common. The akoya dies at harvest. South Sea and Tahitian mollusks (Pictada margaritifera and Pinctada maxima) accept only one nucleus at a time, but as they do not die at harvest, they may be nucleated several times. If a particular mollusk has been successfully nucleated several times and consistently produces fine pearls, the mollusk is often returned to the wild to strengthen the genes of future generations of spat.
Freshwater Pearl Nucleation
Freshwater pearls must also be nucleated, but in a different fashion. In lieu of the mother-of-pearl bead, freshwater pearl farmers nucleate their mussels with small pieces of mantle tissue. These mantle tissue pieces are not placed in the reproductive organ of the mussel, but in the fleshy mantle tissue. Because the mantle tissue is large and located on either side of the shell, each mussel can withstand many insertions. Most mussels receive 12 to 16 insertions on either side of the valve for a total of 24 to 32. The large number of freshwater pearls produced per mussel accounts for some of the diminished value between freshwater pearls and their saltwater cousins. But, because the mantle tissue is dissolved into the pearl-sac, freshwater pearls are solid nacre.
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