What Does Science Tell Us About Pearls?

Pearls sit where biology, chemistry and physics meet. This section explains what science has learned about how pearls are built and how labs read them.

How mollusks build nacre#

Nacre is secreted by the mantle of certain mollusks, some bivalves and some gastropods. It is built from layers of microscopic platelets of aragonite, a form of calcium carbonate, bound together by a fine network of a protein called conchiolin. The same material lines the inside of the shell.¹ The pages nacre and pearl-producing mollusks go further.

Inside a pearl, the cells that make nacre form a closed pocket, the pearl sac. In the black-lipped pearl oyster, the sac grows around a cultured pearl’s bead within 14 days, and the first nacre appears about 32 days after grafting.² The page how pearls form follows the whole process.

Why pearls shine#

A pearl’s luster comes from light reflected off its surface and off the concentric layers of nacre inside it, which GIA compares to light bouncing off a convex mirror.³ The layered structure also produces two optical effects: overtone, translucent color lying over the pearl’s main bodycolor, and orient, a shimmer of iridescent rainbow colors on or just below the surface.^{1 4}

Thin nacre shines less. GIA notes that a bead visible under the nacre, or a dull, chalky look, means the nacre is thin, which affects both luster and durability.⁴

A pearl’s physical properties#

GIA’s gem encyclopedia records these properties for pearl.

What chemistry reveals#

The water a pearl grew in leaves a trace in its chemistry. Freshwater pearls contain more manganese and barium, and less sodium, magnesium and strontium, than saltwater pearls. Labs measure these elements directly, and also through X-ray luminescence, which brightens with manganese.⁵

Color has a chemistry too. A reflectance feature at 700 nm is a key identifying sign of pearls from the black-lipped pearl oyster, the mollusk of Tahitian cultured pearls.⁶ Spectroscopy also helps separate natural golden color from color produced by heat or dye.^{7 8}

What X-rays reveal#

X-ray radiography and X-ray computed microtomography are central to identifying pearls.⁹ A natural pearl shows no bead or curved cavity in its center, and a uniform, onion-like structure.² GIA also uses real-time microradiography to measure the nacre thickness of bead-cultured pearls when the nacre looks thin or a client asks.¹⁰