What Is a Pearl?

Every pearl, whether a natural pearl or a cultured pearl, is grown by a living mollusk. This section covers what pearls are made of, how they form, and the words used for the different kinds.

What a pearl is made of#

A pearl is made of calcium carbonate and organic material, laid down in concentric layers inside a mollusk.¹ In most pearls used in jewelry, those layers are nacre: microscopic platelets of aragonite bound together by a protein called conchiolin. That layered structure produces the optical effects called orient and overtone.²

GIA records pearl’s chemistry as calcium carbonate and its hardness as 2.5 to 3.0 on the Mohs scale.³ French Polynesia’s pearl law puts the nacre of Tahitian cultured pearls at about 92% aragonite and 6% conchiolin, with the rest mineral salts and water.⁴

Natural, cultured and imitation pearls#

Three words cover almost every pearl you will see for sale:

• Natural pearls form inside a mollusk with no human help of any kind, usually around a microscopic irritant.⁵ They can be nacreous or non-nacreous.² The natural pearls section covers them.
• Cultured pearls form after a person starts the process. A technician places a shell bead with a piece of mantle tissue in the mollusk, or tissue alone, and the mollusk coats it with nacre.⁵ Cultured pearl farming now supports industries in more than 30 countries.⁶
• Imitation pearls are manufactured products, made of any material, that only look like a pearl or cultured pearl.¹

In the United States, the unqualified word “pearl” means a natural pearl. A cultured pearl has to be called “cultured” or “cultivated”, and an imitation “imitation”, “artificial” or “simulated”, in letters as clear as the word “pearl”.¹ CIBJO, the international jewelry confederation, requires the same words, “cultured” for cultured pearls and “imitation” for imitations, and recommends labeling natural pearls as “natural pearls”.²

The page natural, cultured and imitation pearls explains the terms and the rules for using them, and natural vs cultured pearls compares the first two side by side. The page pearl myths checks popular beliefs against the evidence.

Is a pearl a gemstone?#

Yes, although it is an organic gem grown by an animal rather than a mineral. GIA lists pearl in its gem encyclopedia,³ and its laboratory has identified and classified natural and cultured pearls since 1949.⁷ Pearl is also a birthstone for June, together with alexandrite and moonstone.⁸ The page June birthstone tells that story.

Which animals make pearls#

Pearls valued as gems come mainly from marine pearl oysters and freshwater mussels. Akoya cultured pearls grow in the Akoya pearl oyster, South Sea cultured pearls in Pinctada maxima and Tahitian cultured pearls in Pinctada margaritifera.⁵ Most freshwater cultured pearls come from China, from mussels such as Hyriopsis cumingii and its hybrids.^{5 9} The pearl types section covers each one.