Pearls: The Unabridged Story
The Unabridged Story of Pearl Formation (and Categorization)
Did you ever, as a child wonder where pearls came from and then how they were made? Later on did you wonder if the oysters that provided pearls survived, or were at least eaten? Was pearl formation explained to you in way that ran as follows (or at least contains this information):
Once upon a time there was an oyster, let's call him Oliver, Oliver O. Oyster, and he lived in a shell at the bottom of the Ocean. One day, Oliver O. Oyster got a grain of sand stuck in his shell. Oliver, irritated by the sand secreted a liquid around the sand to alleviate the itch. After a period of time the secretions built up forming a shiny pearl. Our Oliver O. Oyster remained at the bottom of the Ocean for several more years doing what oysters do until one day a diver happened to spot him. The diver then picked him from the Ocean floor and brought him to the surface. Once back on his boat the diver opened up Oliver Oyster and removed his pearl and throwing him back into the ocean. And that is how we get pearls. The end.
The story of Oliver O. Oyster is only a fraction of the story of how pearls are formed and obtained. I will attempt to provide an unabridged non-narrative version of how pearls are formed and obtained. To start off, the term pearl is applied to two things, 'true pearls' (1 and 2) formed by bivalves (organisms that form two shells, such as oysters and muscles) and 'not-true pearls' formed by univalves (organisms that form a single shell, "snail-like" (1) creatures). 'True pearls' refer to the type the Oliver O. Oyster would make, so lets talk about them first.
True pearls are formed by bivalves, two shelled mollusks found both in salt and fresh water. Based upon this there are two types of pearl, freshwater and saltwater. Both types of pearl can be formed in one of two ways, naturally or cultivated. Natural pearls are formed when an "irritant" (2) becomes lodged in the flesh of a bivalve (forms a "cyst pearl" (1)) or between the bivalve and it's shell (forms a "blister pearl" (1)). In order to protect itself from the foreign mass the bivalve coats it with the same materials used to make for its shell, aragonite and conchiolin (collectively called mother of pearl or nacre). This is why the color of the inside of the shell and the color of the pearl match each other. After a period of time, during which the foreign mass is repeatedly coated with mother of pearl, the object becomes a pearl.
Cultivated pearls are formed in exactly the same way as natural pearls, but the manner of pearl formation is slightly different. Natural pearl formation is caused when an irritant becomes embedded in a mollusk by chance, whereas cultivated pearls are formed due to the irritant being purposefully embedded in the mollusk through human interference. Because of this natural pearls are rarer, occurring in something like "1/1000 or 1/500,000 depending upon the species." (1)
The matter in which the true pearl is obtained depends, of course, in the matter in which it is formed. If the pearl is natural it is found in wild bivalves and thus must be found, on the other hand cultivated true pearls, whether saltwater or freshwater are harvested, because it is already known exactly where the mollusks will be located as they were purposefully placed there. The story of the obtainment of the pearl does not end here, however. In order to get to the pearl the shell of bivalve (and univalve) is pried open and the (uni/bivalve) usually, but not always, dies in the process.
Pearl also refers to 'not-true pearls,' which are "calcareous concretions" (1) formed by univalves, single shelled snail-like organisms such as the conch snail. What I call not-true pearls are technically called "Non-nacreous 'pearls' (1)." These pearls are formed in the same way as true pearls, but instead are "made up of calcium carbonate (which is what Nacreous Pearls are made of) but primarily in the form of calcite rather than aragonite, and with different structural characteristics and protein proportions than their nacreous cousins." In other words while both nacreous and non-nacreous pearls are made up of calcium carbonate, nacreous pearls are made of aragonite and conchiolin and non-nacreous pearls are made up of calcite. This is the unabridged story of the formation and classification of pearls.
While my, and most likely others, childhood story is correct, it leaves out a few facts while getting the technical formation of pearls correctly (minus the misconception that pearls are formed from a grain of sand). For example, oysters make pearls, however, so do other bivalve mollusks. It is also true that some pearls come from the ocean, but they could just as well have come from a lake. I think though, that the story is told in such a way that children can comprehend the information without having to deal with the troubles of purposeful irritation of a fellow creature (cultivated pearls), and without having to come to terms with the fact that in obtaining the pearls most, but not all, of the bivalves and univalves are killed.
1. Gem of the Month: Natural Pearls
2. American Museum of Natural History: Pearls
3. Wikipedia: Pearl
4. Wikipedia: Univalve
5. Wikipedia: Bivalve
6. American Pearl Glossary of Terms
7. The Pearl Page by Jed. G. Archuleta, The Conch Pearls section