Oysters are surrounded by sand and ingest it all the time. Like all benthic mollusks, they are very good at expelling unwanted material from between the shells. A grain of sand entering the body isn’t going to bother it at all. What will bother the mollusk is damage to the mantle.
The mantle of a bivalve is a thin, sheet-like organ that coats the inside of the animal’s shell. Its purpose is to actually create the shell by converting dissolved minerals in the surrounding water into solid calcium carbonate, strengthened by a fibrous protien compound called conchiolin. This material secreted by the mantle is known as nacre, or "mother-of-pearl", and can be seen in the smooth, shiny surface of the inside of any dead mollusk shell (as opposed to the rough, worn outer surface normally seen on the live animal).
The damage to the mantle that causes the oyster to create a natural pearl is never a foreign object like sand or stone, but is most likely caused by a parasite or a symbiont. These organisms occasionally tear tiny bits of flesh from the mollusk and carry it to another part of the body. If this piece of torn flesh happens to be part of the mantle, it will begin secreting nacre, forming what is known as a pearl sac over the parasite. This can happen anywhere in the oyster’s body and the animal’s response to the pearl sac is to continue to secrete more nacre over it. This is then repeated, layer upon layer for several years, resulting in the formation of a pearl. Notice that in this case there is no "seed" at the center of a pearl (unless you include the long dead parasite itself). It is pure nacre.
The conditions required for this to happen naturally are exceedingly rare. I’ve been working around marine animals since I was twelve, and in thirty years I have seen exactly one natural pearl. In a slipper limpet, of all things. So to accommodate the jewelry industry’s need for these gems they have to be created artificially in pearl farms, mainly found in China, Japan and Southeast Asia.
Artificial pearls are formed not just by inserting a stone or shell fragment between the bivalve’s shells, as is commonly believed. First you start by cutting out a tiny piece of mantle tissue from a donor oyster. This tissue is attached to a bead of mussel shell and placed into the gonad of the host oyster. The foreign tissue serves as a graft and the bead will be the pearl’s nucleus. Within hours the graft begins secreting epithelial cells around the bead, these epithelial cells start producing nacre and the pearl formation process has begun. A few years later the pearl can be harvested. In some species the pearl can be extracted without killing the host. In fact, the animal can even be reseeded and released back into the oyster farm to continue manufacturing its product.
To produce marketable pearls the nucleus needs to be perfectly round since the pearl will take the shape of the seed object. If the seed particle isn’t round (and sometimes it isn’t by design) the result is an irregular shape known as a baroque pearl. Most natural pearls, not having a nucleus to guide its shape, are baroque. Perfectly round natural pearls are extremely rare and, of course, very valuable. Interestingly enough, all commercial pearl oysters are Pacific species, but the seed shell used as a nucleus are obtained from freshwater mussels exported from the Mississippi River. For some reason, beads made from this American species create the highest quality pearls.
In some species of freshwater pearl mussels this shell nucleus isn’t even needed. Simply inserting the donor mantle, or even making a small slice in its own mantle, will initiate pearl formation. The advantage of using the seed particle is to control the shape the pearl eventually takes. Also, these non-nucleated pearls may take eight to ten years to grow large enough to be commercially useful. Periculturists (that’s what pearl farmers are called) can speed up this process greatly, producing salable gems in less than six months, by inserting a large shell bead with the mantle graft into the oyster and simply allowing the epithelial cells produced by the mantle tissue to coat it with a quick layer or two of nacre. Instant pearl, just add water.
Historically the pearl industry consisted of pearl divers in the tropics collecting wild oysters from the sea bed. These were then opened on shore to check them for pearls, killing the mollusk in the process of course. No commercial pearls are produced this way any longer. This is primarily because of the ease of cultivating them artificially as well as guaranteeing that each mussel or oyster contains a gem. In even the best pearl producing species only one natural pearl will be found for each ton of oysters collected. The few pearl diving operations that exist today, mainly in Australia, collect the animals for culturing rather than searching for the natural pearls themselves. To give you an idea of just how rare and valuable a perfectly round natural pearl is, the Roman general Vitellius once payed for an entire military operation by stealing one of his mother’s pearl earings and selling it to a wealthy landowner.
Although mass-producing pearls artificially is a fairly recent development, a similar process was once used many years ago. As early as the first century C.E the Chinese were inserting tiny Buddha-shaped molds into freshwater mussels to produce venerated Buddha-shaped pearls. These Buddha formations were not actually free standing round structures we usually think of as pearls, but were "half-pearls" known as mabes, which grew attached to the inside of the shell. Still, these mabes grew in the shape of the fat guy.
Finally, here’s how imitation pearls are formed. These knockoffs aren’t even made inside a mollusk, but instead are simply rounded and polished pieces of conch shell or coral. The really cheap stuff are simply glass beads that are coated in a solution called essence d’Orient, which is a frenchified way of saying "herring scale oil". The guanine crystals that give fish scales their luster are extracted from the scales and used to coat the glass beads and give them a resemblance to actual mother-of-pearl. It takes a hundred tons of herring to extract one ton of herring scales in order to produce a single pound of essence d’Orient.
Fun Fact: The largest pearl ever found is called the Pearl of Lao Tzu. It weighs over fourteen pounds and was discovered in 1934 inside a giant clam off the coast of the Philippines.
Other diaries in this series can be found here.
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