How a Pearl Is Built, Layer by Layer

A pearl is the only gem in the usual jewellery cabinet that an animal makes. No one facets it out of a rough crystal. A mollusc, an oyster or a mussel, lays down a smooth coating inside its shell. When that coating builds around a small body in the soft tissue, the result can be a pearl.

What the coating is

The coating is called nacre. It is mostly calcium carbonate, in the crystal form aragonite, glued by a small amount of organic material. The layers are thin. Light reflecting between them is what gives a pearl its soft glow, called orient. A bead of glass can be round. It does not have those layers, so it does not glow in the same way.

The same nacre lines the inside of the shell. That lining is mother-of-pearl. A pearl and a piece of shell inlay are related, but they are not the same object. Inlay is cut from the shell. A pearl grew in the tissue.

Natural and cultured

A natural pearl starts without a person placing anything in the mollusc. They are uncommon in jewellery today because they take chance and time, and many old beds were fished hard.

A cultured pearl starts when a technician inserts a nucleus, a bead or a piece of mantle tissue, into the mollusc. The animal then does the same work it would do for a natural pearl: it covers the insert with nacre. The round pearls most people see in shops are cultured. That is not a deception if it is said plainly. It is the normal method of the last hundred years.

Freshwater pearls, often from mussels in lakes and rivers, and saltwater pearls, from oysters in the sea, can both be cultured. The name of a famous bay, or the word “South Sea”, describes a type of cultured pearl and a region. It is not, by itself, a laboratory grade.

Shape, surface and thickness

A pearl can be round, oval, drop-shaped, or baroque, which means irregular. Round is harder to grow, so it often costs more. It is not more “real”.

Look at the surface in a normal light. Fine ridges, small pits and dull patches are common. A perfectly featureless, very cheap ball is a reason to ask whether you are looking at nacre at all, or at a coated imitation.

Nacre thickness matters on bead-nucleated pearls. A thin coat can wear through at the drill hole and on the high points. A laboratory report, when you have one, may comment on the identity of the pearl and on treatments. A shop’s word should match that report. If there is no report, the bill should still say natural, cultured, or imitation.

Care

Pearls are softer than sapphire or diamond. They are also organic. Perfume, hairspray and acid (including vinegar) can dull them. Put pearls on after scent, and wipe them with a soft cloth. Store them away from harder stones that can scratch the nacre. Do not soak a strand. The silk thread can rot.

Traditional texts link pearls with the Moon. That is a cultural association. It is not a medical result, and it does not change how the pearl was grown. Judge the pearl by its identity, its surface, and the words on the bill.