Kuoret ovat nilviäisten tuottamia komposiittimateriaaleja, joissa yhdistyvät ympäristöstä peräisin olevat epäorgaaniset mineraalit (CaCO3）) ja orgaaniset aineet, joita nilviäiset itse tuottavat ympäristön lämpötilassa ja paineessa. Tämä prosessi on eräänlainen orgaanisen aineen säätelemä biomineralisaatio. Joissakin kuorissa, erityisesti helmiäisostereiden kuorissa, on helmiäiskerros, joka tunnetaan nimellä "nacre" ja joka on koostumukseltaan ja rakenteeltaan samanlainen kuin helmet.

The pearl layer is formed under the control of organic matter secreted by the mantle cells of mollusks. It begins with the secretion of an organic framework by the mantle, and inorganic ions and proteins secreted by the epithelial cells on the mantle seep out in the form of calcium carbonate colloidal droplets through the pores of the mantle protein layer within this framework. As it gradually grows, expands, thickens, and extends, the upward growth stops when it is hindered by the upper layer of the shell material; it then develops laterally, becoming flat until adjacent crystals restrict it. This results in the orderly arrangement of aragonite microcrystals within the pearl layer, resembling a mosaic, as well as the structural characteristics of shell material distributed in its gaps, and the calcium carbonate layer also gradually grows, expands, thickens, and then grows laterally in a flat manner.

The theories regarding the formation of the pearl layer mainly include the following:

(1) The age theory of the epithelial cells of the outer mantle

Since the edge of the shell is composed of calcite prism layers. In contrast, the inner side is composed of the pearl layer, and the epithelial cells at the outer edge of the shell (corresponding to the position of the prism layer) become older as they move inward into the shell.
The younger columnar cells at the outer edge of the epithelial layer of the mantle are related to the prism layer; the older cuboidal epithelial cells on the inner side are related to the formation of the pearl layer.

(2) Intracellular crystallization and extracellular assembly theory

This theory posits that the outer membrane cells secrete organic matter, ions, and other shell precursors, which crystallize and precipitate in the outer cavity between the membrane and the outer shell layer through a series of interactions to form the shell. Low-density calcium particles exist in the vesicles of the epithelial cells outside the membrane; in the initial pearl layer on the inner surface (facing the membrane), the structure of the pearl layer is quite imperfect, with poor orientation. Still, the entire pearl layer is highly oriented.
The vesicles in epithelial cells serve as the initial nucleation sites for carbonate minerals in the pearl layer, where calcite prisms and aragonite tablets are formed and then transported by vesicles to the outer cell surface to assemble into the calcite prism layer or aragonite pearl layer of the shell.

(3) “Compartment” theory

This theory suggests that organic matter pre-forms compartments, where crystals nucleate and grow, and the compartments’ shape restricts the crystals’ shape.
The organic matrix secreted by the mantle forms small compartments. In the compartments, acidic groups bond with calcium ions, increasing crystal growth. When the crystals encounter the organic fiber “plates” vertically and adjacent crystals horizontally, their growth stops, ultimately forming the layered structure of the Nacre.

(4) “Mineral Bridge” theory

This theory posits that the structure of the Nacre is formed through the continuous growth of “mineral bridges.” Each “mineral bridge” is essentially cylindrical, with a height equal to the thickness of the organic matrix layer. Crystals can continue to develop on already formed crystals, possibly interspersing through the pores of the matrix between micro-layers and forming the nacre layer through interstitial deposition. Further research on “mineral bridges” has revealed their geometric features and distribution patterns within the organic matrix layer, suggesting that the microstructure of Nacre should be described as a “brick-bridge-mud” structure, where the double-shell nacre layer does not have pre-formed compartments; the “compartments” are merely an illusion. When crystals come into contact with other crystals as they grow, organic matter naturally gets trapped between them.

Aragonite crystals continue to grow through the organic plates’ pores between layers. Each newly nucleated aragonite plate grows vertically towards the mantle until it encounters another layer of interstitial matrix plates, at which point the vertical growth will stop, and then the plates will grow laterally to form new plates. In stacked nacre layers, the vertical growth rate is about twice that of lateral growth, indicating that a newly nucleated plate grows fastest along the c-axis. Once the growing plate encounters the pores in the adjacent interstitial matrix above it, it will pass through the pores like a mineral bridge, allowing a new plate to crystallize; this new plate has a lateral offset relative to the lower plate. As the older plate grows laterally, more mineral bridges are formed between the new plates, allowing the plates to grow simultaneously in more locations. However, the first mineral bridge plays a crucial role when nucleating new plates.