I learned this today. A pearl forms in the shell of a mollusk when an irritant becomes trapped there and the mollusk covers it with a fluid that hardens.
There are three different types of pearl. There are natural pearls, cultured pearls, and imitation pearls. I am not going to look at imitation pearls. Natural and cultured pearls produce the pearl in the same way. The only difference is that the irritant is manually inserted into the shell in the case of the cultured pearl. Natural pearls are obviously more expensive than cultured pearls because of their rarity, but even among natural pearls, there are different types that have different values.
Pearls are usually formed in oysters, mussels, or clams, but they can also be found in other mollusks.
Let’s look at natural pearls first. They start to grow when a microscopic intruder or parasite gets into the shell of the shellfish. To protect itself, the mollusk forms a pearl sac around the irritant. The pearl sac is made from external mantle tissue cells. The oyster then secretes aragonite and conchiolin to cover the pearl sac.
Conchiolin is a complex protein that is secreted by the outer layer of the mollusk. It binds together with aragonite to make the mollusk’s shell. If you open an oyster shell, the inside very often looks like a pearl. That is because it is made of the same stuff.
Once the irritant is covered, the oyster repeats this process, and the pearl grows larger and larger. The longer it goes on, the larger the pearl is, but they are not always perfectly round. Pearls can grow in many different shapes and saltwater pearls tend to be of a more consistent shape than freshwater ones. Freshwater pearls are often irregular in shape. Some pearls can be worth more because they are not perfectly round.
Pearls and the inside of the oyster shell have a very distinctive shine and color. This is known as “mother-of-pearl” and it is caused by nacre.
When the oyster starts to cover the irritant in the pearl sac, it secretes calcium carbonate (calcite or aragonite) from the inside of the sac and then from the outside of the sac. It makes a brown pebble like ball. Then the aragonite on the outside of the brown ball starts to form nacre.
Nacre is made from hexagonal platelets of aragonite that are joined together by layers of conchiolin. The microscopic platelets build up on top of each other and each layer overlaps the platelets of the layer below, in the same way a wall of bricks is built. This gives it two very interesting characteristics. Firstly, it makes the pearl incredibly strong and durable. Secondly, it gives the beautiful glossy sheen and the mother-of-pearl color. This is caused because the layers break up light that falls on their surface. The light enters the gaps between the aragonite and bounces around before being reflected back. It is broken up, reflected, and refracted in such a way that the nacre appears to be a different color depending on the angle it is viewed from. The beauty is a mere by-product of the material that has probably evolved to be the way it is for its strength.
Natural pearls are very rare. It is estimated that only about 1 in 10,000 wild oysters will yield a natural pearl. This is why they are so expensive.
Cultured pearls can also be expensive and they are not always guaranteed to form, but the odds are better than those of natural pearls. With a cultured pearl, a tiny piece of shell from a donor shell is implanted as the irritant. Sometimes a round bead is used. The oyster covers the round bead in the pearl sac and the resultant pearl is likely to follow the shape of the bead and be round.
There is no guarantee that a cultured pearl will grow. Many of them will be misshapen, the irritant might be rejected, or the oyster might die from disease. Only about 5% of pearls produced on a pearl farm are of high enough quality to go to the market. Even so, they can still be produced at greater rates than natural pearls.
Cultured pearls can be ready in as little as a year, while it can take over three years for a natural pearl to grow. This is after the three years it takes for the oyster to reach maturity. Another difference is that more than one cultured pearl can be grown in an oyster at any one time, greatly increasing the yield.
The introduction of the cultured pearl brought the price of pearls down and harmed the pearl industry for decades. When it became possible for anyone to own a pearl necklace, the people who could afford natural pearls, no longer wanted one.
So, a pearl forms when the oyster or mollusk secretes a layer around an irritant to protect itself. And this is what I learned today.
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Photo by Marta Branco: https://www.pexels.com/photo/white-pearl-ring-on-blue-surface-1395305/
Sources:
https://www.jthomasjewelers.com/pages/how-pearls-are-formed
https://www.amnh.org/exhibitions/pearls/what-are-pearls/how-pearls-form
https://en.wikipedia.org/wiki/Pearl
https://www.nhm.ac.uk/discover/quick-questions/how-do-oysters-make-pearls.html
https://en.wikipedia.org/wiki/Conchiolin
https://en.wikipedia.org/wiki/Nacre