Mother of pearl, also called nacre, is an iridescent layer of material that forms the shell lining of many mollusks. Pearl oysters and abalone are both sources of this substance, which is widely used as an inlay in jewelry, furniture, and musical instruments. Mother of pearl comes in several natural colors, but is often bleached and dyed for decorative use. The dye retains the shimmering layers which make the material so sought after. Nacre is a tough and resilient material, but it is relatively soft and easily scratched.
Two substances, one mineral and the other organic, combine to create mother of pearl. Tiny hexagonal plates of aragonite, a form of calcium carbonate, are arranged in layers alternating with conchiolin, a flexible protein similar to silk that is secreted by the mollusk. Aragonite on its own is very brittle, but combined with the protein it forms a strong, flexible material that can withstand hard use.
The mollusk first of all secretes a layer of conchiolin. Aragonite crystals then form on this surface at numerous points, growing until they meet each other to form tiny plates. A further layer of protein is then deposited and so on, so that over time, many layers build up.
The resilience of mother of pearl comes from its layered structure and the contrast in properties between the mineral and organic layers. The aragonite consists if hard, brittle crystals, while the organic protein layers are a natural polymer that is flexible and resistant to fracture. The aragonite gives relative rigidity to the material and the protein counteracts the brittleness of the aragonite by preventing the spread of fractures. The result is a natural product that is tough and resilient. As of 2013, research is underway into the manufacture of synthetic nacre.
Mother of pearl has a hardness of about 3.5 on Moh’s scale. This is quite soft compared with gemstones and most metals. Although its relative softness makes it easy to work with and cut into shapes, it also means that nacre objects can be easily scratched by other items of jewelry, for example.
The iridescence that makes nacre so attractive results from the fact that the aragonite crystals have a range of thicknesses that are close to the wavelengths of visible light. Some of the light that strikes the crystalline layers will go through to the layers below, while some is reflected from the upper and the lower surfaces. Layers tend to reflect particular wavelengths, that is, colors, depending on their thickness and on the angle of the light. When viewed from different angles, different colors are reflected. Small irregularities on the surfaces make every piece of nacre unique.
Mollusks create mother of pearl to protect themselves. In addition to forming part of the shell, it also insulates mollusks from bacterial infection and parasites. Another function of this substance is to reduce irritation or damage from material such as sand or grit that drifts into the shell. The particle is gradually surrounded by layers of nacre, rendering it harmless. This may result in a blister-like irregularity on the inside of the shell, or it may create the unattached, spherical structure that is much prized by humans as a pearl.
The pearly lining of mollusk shells has long been noted by people living near the ocean. Many early cultures used it extensively in jewelry and it came to be highly prized. Although not as popular as it once was, many modern cultures still appreciate its beauty and mother of pearl continues to be used in jewelry, ornaments, furniture inlays and musical instruments. Some homes integrate it into tiles and other fixtures, although it is no longer used extensively.
Like other substances found in nature, mother of pearl develops irregularities as it forms. As a result, every piece of jewelry or inlay is slightly different, a fact that can add to its appeal. Artisans may work with the unusual features of a specific piece to highlight them. These irregularities may also appear in cheaper jewelry that is not as meticulously constructed.
Nacre has been much studied by scientists interested not only in its strength and durability, but also in what it can tell them about the conditions in which it formed. The thickness and structure of the layers secreted by marine mollusks depends on the temperature and water pressure. It is therefore possible to obtain valuable information about past conditions from the study of fossilized material.
Mother of pearl items should not be stored with anything that might scratch them. They should be cleaned with mild soaps and water; harsher cleaning agents may react with the aragonite, especially if they are acidic. This also applies to pearls, which have an outer coating of nacre.