The Nucleus: Heart of the Pearl

Around here we’re pretty obsessed with pearls, and we love to be able to educate our customers all about these wonderful little gems. Pearls come right out of the mollusk in perfect condition, with no need for polishing or faceting. But how? How exactly does an oyster produce a pearl? And what’s this term “cultured pearls”? If they are from nature, how does a human make a pearl? There are actually quite a few answers to this question.

At the heart of the pearl is the nucleus, which is a natural material that irritated the oyster. Contrary to the commonly held misconception that a pearl is begun by a grain of sand, in natural (sometimes called “oriental”) pearls, the nucleus is usually a parasite that has drilled its way through the shell and into the mantle, which is the oyster’s outer organ. This irritates the oyster, so the animal begins to layer cells from the mantle around it, forming what is called a pearl sac.

Anatomy of an oyster
Anatomy of an oyster, showing the mantle and where the pearl is ultimately formed.

The next step in pearl production is  the layering of an egg-white-like substance called nacre onto the irritant. Made up of a protein called conchiolin, along with minerals the oyster absorbs from the water nearby, nacre is the same material found in mother-of-pearl. The conchiolin in the nacre acts as a glue for layers of calcium carbonate and aragonite, and layers of this nacre is what creates the pearl’s natural color and luster.

Mother of Pearl with Drill Holes from Parasites
On this piece of mother of pearl, you can see multiple drill holes where parasites attempted to enter the oyster.

The pearl culturing process is similar to the natural formation of pearls, except a human surgically implants a nucleus into the oyster. In saltwater oysters, a small bead made from polished shell from another oyster is used. This is inserted into the oyster’s reproductive organ along with a piece of mantle tissue from a donor oyster. The cells from the outer layer of the mantle then form a pearl sac around the bead nucleus. After the pearl sac is formed,nacre is secreted in layers around it, forming what we know as a pearl.

Examples of shell bead nuclei for pearl production
Different examples of the shell bead nuclei used in saltwater pearl production. The shell on the left is from the Mississippi freshwater mussel.

Research by Kokichi Mikimoto and his successors in the pearl industry has determined that the best shells for starting a new pearl come from North American freshwater mussels, because of their thicker shells. These are primarily farmed in Mississippi and Tennessee. The long term supply of these shells is uncertain, which creates a high price for the shell beads, especially in the larger sizes. This is another contributing factor to the higher price of saltwater pearls.

Here are some really interesting cross-section photos of cultured saltwater pearls, showing the shell bead nucleus and layers of nacre:

Cross Section of a Tahitian Pearl
A tahitian pearl. You can clearly see the layers of dark nacre around the shell bead.
The layers of nacre around this South Sea Pearl are almost gold in color.
The layers of nacre around this South Sea Pearl are almost gold in color.

The process for culturing freshwater pearls is actually very different. The primary method for culturing freshwater pearls actually does not use a nucleus. A small piece of mantle tissue from another oyster is implanted into the mantle of the pearl oyster. This piece of tissue grows and feeds off of the mantle it is grafted into, but eventually it becomes difficult for the tissue to get nutrients from the pearl oyster, and the transplanted tissue dies. The result is that the hardened tissue eventually prompts the formation of the pearl sac and allows a pearl to form.

12-50 implants can be done in this way, depending on the species of mollusk. This accounts for the large amount of freshwater pearls on the market. In fact, freshwater pearls (particularly from China) make up around 90% of the pearl market. Surprisingly, this method usually creates more perfectly round pearls than methods using nuclei. You can use nuclei to culture freshwater pearls, but round nuclei usually create baroque pearls due to the higher production of nacre in freshwater oysters.

A freshwater pearl that has been cut in half.
This photo of a freshwater pearl that has been cut in half shows you the difference between the nucleus used to culture a freshwater pearl vs. the beads used to culture saltwater pearls. The nucleus of this pearl (the bright white line in the center) is much smaller and has not been polished into a bead shape. The freshwater pearl’s nacre is much thicker than with a saltwater pearl, and you can see several colors of nacre. This final pearl is a gunmetal grey color and you can view it at Pearls International in Daytona Beach Shores.

Flat nuclei create coin pearls, long, thin nuclei create stick pearls, and so on and so forth. The huge variety of  shapes seen in freshwater pearls are not usually found in saltwater pearls.

Strands of freshwater pearls in unusual shapes.
Strands of freshwater pearls in unusual shapes.

sources: www.kojimapearl.com/about-the-pearls/freshwater-pearls/, www.sustainablepearls.org/sustainability/sourcing-nuclei/, gemologyproject.com/wiki/index.php?title=Pearl

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