What Is Nacre, or Mother-of-Pearl?

Nacre is the material most natural and cultured pearls are made of, and the source of their glow. The same material lines the inside of the shells that produce them.¹

What nacre is made of#

Nacre is built from layers of microscopic platelets of aragonite, a form of calcium carbonate, bound together by a fine network of an organic protein called conchiolin. It is secreted by the mantle of certain bivalves and some gastropods.¹

It is mostly mineral. 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.²

Nacre up close

• Flat platelets of aragonite
• Thin organic layers of conchiolin between them

Nacre over a bead

• The oyster grows only the outer nacre
• Nacre thickness is measured from the bead to the surface

Text version of this diagram

The first drawing shows nacre greatly magnified: rows of small, flat aragonite platelets stacked like bricks in a wall, each row offset from the one below, with a thin colored line between the rows for the organic layer of conchiolin. The second drawing shows a bead-cultured pearl cut in half: a large shell bead in the middle, a band of nacre rings around it, and a bracket across the band marking the nacre’s thickness, from the surface of the bead to the surface of the pearl.

Almost any mollusk can produce a pearl-like object, which scientists call a calcareous concretion, but the pearls valued as gems come from species that secrete nacre.⁴

How nacre gives pearls their glow#

A pearl’s luster comes from light reflecting off its surface and off the concentric layers of nacre inside it.⁵ CIBJO’s guide explains that when the aragonite crystals are large, dense and stacked regularly in many layers, light produces the deep luster characteristic of pearls, and that luster is closely related to the thickness of the nacre.⁶

The layers also make color. Light interfering in the nacre’s thin layers produces the iridescent orient of some pearls,¹ and depending on the thickness of the layers, a pearl may show pink or green interference colors.⁶ The page pearl color explains more.

Polishing can make a pearl’s surface shine like a mirror, but CIBJO notes that this improves only the reflection from the surface, not the deep, soft luster that comes from within the nacre.⁶ The page luster explains how it is graded.

Pearl or mother-of-pearl?#

Mother-of-pearl is the smooth, hard, iridescent lining of the inner shell, which scientists call nacre.¹ A pearl is a separate formation, laid down layer by layer inside a pearl sac within the mollusk’s body.¹ Usually, but not always, natural pearls have the same color, composition and general quality as the mother-of-pearl of the mollusk that made them.¹

Mother-of-pearl has uses of its own. CIBJO’s guide rates the mother-of-pearl of Pinctada maxima above that of any other pearl oyster for quality, thickness and value, and says it supplies most of the world’s demand for products such as watch faces and jewelry. The black-lipped oyster’s dark mother-of-pearl is used for inlay and buttons, while the shells of the freshwater mussels used for pearls in China have little luster and no ornamental value.⁶ In Western Australia, the shell is sold for jewelry and buttons alongside the pearls.⁷ Abalone shell is used as mother-of-pearl too, with nacre in many hues of blue, green, cream, red and purple.¹

Nacre thickness#

On a bead-cultured pearl, the oyster grows only the nacre over the bead, so its thickness matters. CIBJO defines nacre thickness as the average thickness of the nacre over the bead, in millimeters. It applies only to beaded cultured pearls, and it is not the same as nacre quality.¹

Nacre thickness is closely related to how long the pearl was cultured, and it can affect the pearl’s color, luster and durability.¹ GIA says a bead visible under the nacre, or a dull, chalky look, means the nacre is thin.⁸

It varies widely. In one X-ray image GIA published, a 6.85 mm Akoya had about 0.34 mm of nacre and an 11.15 mm South Sea pearl about 1.92 mm.³ The typical range for South Sea bead-cultured pearls is between 1.5 and 3.0 mm.⁹ French Polynesia required at least 0.8 mm on Tahitian cultured pearls from 2005 until its 2017 pearl law made the check voluntary.^{2 10}

Labs measure nacre thickness with X-rays. GIA uses real-time microradiography when the nacre looks thin or a client asks.¹¹

Pearls without nacre#

Not every pearl is nacreous. Clam, conch, Melo, scallop and some pen pearls are natural pearls without a nacreous surface.¹ A conch pearl, for example, is calcium carbonate arranged in a crossed lamellar structure, which usually gives it a flame-like surface pattern and a porcelain-like sheen.¹ The page pearl-producing mollusks covers these pearls and the animals that make them.