Shell Style

Shell, as a word, suggests an exterior-interior relationship. We palm the patterned forms—striped pink, mottled brown, tan striations—in our hands and pry them open to wear pearls around our necks and eat mussels for dinner. A layer between the exterior and interior, in the mantle of the shell, has also held human fascination: nacre. 

Entire shelf of polished shells, revealing nacre's iridescent layer

The outermost layer of the shell is made up of a hard layer of dark protein. Nacre is an iridescent layer is visible on the inside or when the outer layer is polished off, such as the shells in the drawer above. 

Nacre is a composite of aragonite crystals (a type of calcium carbonate) and a membrane-forming protein called conchiolin. The layers of aragonite are close to the wavelength of visible light, so when they interfere with different wavelengths of light at varying angles, it results in the structural colors that make up iridescence. The build-up of aragonite layers creates variation in the colors of nacre—pink, aquamarine, silver, iris. Pigments in the binding regions of the aragonite layers are also integrated into the biomineralization process and contribute to the amalgamate of colors. 

In addition to its iridescence, the layered arrangement of aragonite makes it durable—stronger and lighter than concrete. Research into nacre as a building materials generated synthetic nacre made by artificial calcium carbonate precipitation, but natural nacre is still stronger and more iridescent because of its thicker layers. 

Humans have sought after nacre's iridescent material for hundreds of years, incorporating it as inlay and accessories in architecture, instruments, and fashion. The buttons and pendant below were both made from mollusks of the Gastropoda class (Haliotis refers to abalone).

Nacre's more commonly known as mother of pearl. When an irritant—typically a food particle or microorganism (to contrary belief, a grain of sand)—gets trapped between the shell and tissue, the animal secretes aragonite and conchlion around the particle. Over time, the layers build up into a pearl. Not all of the factors that shape the pearl are inside the shell. Water conditions, like the mineral content of lime and calcium, contribute to the rate of growth. Faster rates of growth are 1-2mm per year. 

Occasionally larger items, like the piece of straw below, will become lodged inside of the shell. The mantle still secretes nacre over it, and it becomes part of the entire shell. 

This particular shell is not nacreous, giving a more porcelain-like interior.
It still secretes a calcerous layer that coats particles.

The physical process that results in nacre and pearls is appealing from a renewal point of view: an object is taken in, and overtime, transforms into a beautiful being (somehow making more sense of abundance of the religious iconography laden in abalone). The transformation isn't an isolated one though—plenty of factors in, out, and between the shell itself and the environment contribute to the beauty that we seek to carry with ourselves.