Pearl Formation

Any species of mollusk that is capable of secreting a shell can also create pearls as the latter are formed through the same physiological processes and materials as the shell of the mollusk. However, not all mollusks secrete pearls that are jewelry grade.

The pearl is, essentially, a defense mechanism that certain mollusks use to remove a potential threat such as a parasite, or any other irritant, such as bits of bone, corral or other foreign objects that have been lodged inside its shell, or even a tumor. The shellfish releases a secretion, known as nacre, which is meant to seal off the intruder from the mollusk to prevent any damage. This secretion is produced by cells of the mollusk’s outer mantle.

Nacre is essentially a calcium carbonite held together with conchiolin, a complex protein that gives a mollusk’s shell its hardness. The concept that a grain of sand is the main irritant that is then transformed into a pearl is mistaken as more often than not the irritant is organic in nature. A grain of sand can be expelled and is considered less threatening than an organic object such as a parasite.

The organic component, in other words the former irritant, usually makes up less than 1 percent of the content of the pearl. However, this organic component determines the orientation of the calcium carbonate that the mollusk secretes, the crystallographic properties of the pearl as well as its color.

Once the irritant enters the mollusk, the latter encloses the particle in what is known as a pearl sac to separate the intruder from the flesh of the shellfish. This sac is made up of cells that are also capable of secreting nacre themselves, thus, if the intruder is successfully surrounded by this sac, the latter begins to secrete shell components. Thus, each new layer will form a new outer layer of shell producing cells, which is why a transversal analysis of a pearl shows that they have a similar structure to onions, with thin concentric layers of nacre and organic matrix.

If this irritant is trapped near the inner lining of the shells then the cells of the outer mantle will surround the irritant with layers of nacre and thus lead to the creation of a blister pearl.