How Pearls are Formed:

The Pearl Sac Theory.

Most people think (or have heard) that a small grain of sand is responsible for the formation of a pearl, so, in order to produce a cultured pearl you got to get a grain of sand inside the oyster. This is way far from being the truth.

A natural pearl is never formed when a grain of sand gets inside the oyster. Pearl Oysters live on the bottom of the ocean, where all the sand you want can be found... they are actually capable of getting rid of sand, bits and pieces of shell, corals, and little pebbles.

The real reason behind the natural formation of a pearl lies behind a biological intruder: a parasitic organism. These may be drilling worms (polychaetes, such as Polydora), a drill-mussel (like Teredo) or many other kinds of organisms (like the drill-sponge Cliona or even the cysts of worms). When a parasite drills thru the shell of the pearl oyster, the oyster becomes irritated and uses its only means of defense to form a barrier: the mantle. This unique organ covers all the inside of the shell of the oyster and is responsible for the secretion of the pearly substance know as mother-of-pearl or nacre. The oyster will use its mantle to secrete nacre on top of these nasty intruders and, if lucky, will coat them and turn them into natural pearls. Very few natural pearls will be of good size (bigger than 8 mm) and of good quality, and those that do are able to fetch very high prices.

The graphic above is a representation of the mother-of-pearl shell (in blue) and the mantle tissue in the oyster (represented by the gray bar between the red colored "cells") as it forms the pearl. The parasite is represented as a dark red spot. What happens here is that the parasite drills thru the shell, it is covered by the nacre secreting cells of the mantle, and finally becomes "engulfed" within the mantle, where it begins to grow... this is the birth of the pearl.

Cultured pearls, on the other hand, are formed not when an intruder attacks the oyster, but when a specialized technician puts one small sphere of mother-of-pearl shell (these come from freshwater mussels that are harvested from the waters of the Mississippi river in the United States) and a small piece of mantle tissue (the "graft") are put together inside the pearl oyster's gonad. This may seem a very different procedure, but the basis is exactly the same as the one depicted above.

The striped-circle in this diagram represents the shell nucleus (the bead or core) and the small thing attached to it represents the "graft" or piece of mantle tissue. Surrounding these is the pearl oyster's gonad tissue. In a few days the piece of mantle tissue will partially degenerate, but the nacre producing (epithelial) cells will start growing around the bead. Later it will start the secretion of nacre, and the cultured pearl will start growing.

A pearl oyster (like the Pinctada mazatlanica) can secrete 3 or 4 concentric layers of nacre each day. These layers measure an average of a thousand part of one millimeter (or, one micron), so they are very thin indeed. It will take an oyster some two years just to coat the bead with at least 1 mm of nacre.

A good quality cultured pearl should have at least 0.8 mm of nacre. If the thickness of nacre is lower than 0.8 mm you risk losing your pearl just by using it (most Japanese Akoya pearls have around 0.1 to 0.4 mm of nacre) or even by sneezing! Avoid such pearls and invest wisely: buy Sea of Cortez Pearls, or what can be described as South Sea Pearl Quality.

Copyright 1999-2002 Perlas del Mar de Cortez

Last Updated 02/2005

All Page Design, text, photos and content by Douglas McLaurin.