Mother-of-pearl is the smooth, iridescent layer that lines the inside of many mollusk shells. It is the same material, nacre, that makes up the outer layers of a nacreous pearl. Pearl oysters, freshwater mussels, abalone and trochus snails all supply it. People have used it for thousands of years in inlay, buttons, beads, carvings and, more recently, watch dials.
What mother-of-pearl is
Nacre is made of tiny, flat tablets of aragonite, a form of calcium carbonate, held together by a thin organic protein called conchiolin. The tablets are stacked in layers, a structure often compared to bricks and mortar. Each tablet is about half a micrometer thick, close to the wavelength of visible light.
When light strikes these layers, some reflects from each surface. The reflected waves interfere with one another, which produces the shifting rainbow sheen called orient or iridescence. The page on what pearls are made of explains this structure in more detail, and gemstone optics covers iridescence alongside other optical effects.
The protein layers also make nacre tough for its hardness. Cracks have trouble passing through the stack, so mother-of-pearl resists breaking better than solid aragonite would. Materials scientists have studied nacre for this reason.
Sources
Many shell-bearing mollusks produce nacre, but only some make a layer thick and attractive enough for use.
- Pearl oysters: the large Pinctada maxima of Australia and Southeast Asia gives white and gold-lipped shell. The black-lip oyster, Pinctada margaritifera, gives darker gray shell with green and pink tones. Both are also used to grow South Sea pearls and Tahitian pearls.
- Freshwater mussels: river mussels in North America, Europe and China supply white to pinkish shell. Chinese pearl farms produce large amounts of shell as a byproduct.
- Abalone: sea snails of the genus Haliotis give some of the most colorful nacre, with blues, greens, purples and pinks.
- Trochus: a top-shell snail of the Indo-Pacific, long used for buttons.
- Turban shells and other sea snails supply smaller amounts.
Shell is often a byproduct of the meat or pearl industries. Some species are protected or harvested under quotas, so supply varies.
Abalone and paua
Abalone shell shows bright, swirling colors. Its nacre layers are often wavy, which adds to the play of color. The species found in New Zealand, Haliotis iris, is known by its Maori name paua. Paua shell shows deep blues, greens and violets and is widely used in New Zealand jewelry and inlay. New Zealand manages paua harvesting through quotas and rules for recreational gatherers.
Abalone can also form pearls. These are usually baroque in shape, very colorful and rare. Cultured abalone blister pearls, which grow attached to the shell, are produced in small numbers.
Uses in inlay and decoration
Mother-of-pearl is cut into thin pieces and set into wood, lacquer, stone or metal. This inlay work has a long history in many places:
- Korean lacquerware with mother-of-pearl inlay, known as najeonchilgi, has been made for many centuries.
- Chinese and Japanese lacquer artists used thin shell pieces in furniture and boxes. In Japan, the technique is called raden.
- Workshops in Syria and the wider Levant are known for furniture and boxes inlaid with shell.
- Musical instruments often use shell inlay on fretboards, headstocks and rosettes.
- In parts of the Pacific, shell was carved into ornaments and exchange valuables.
Carved shell cameos, often from helmet or conch shells, are related work, though they use the layered outer shell more than nacre.
Buttons
Shell buttons were a major industry in the 19th and early 20th centuries. In the United States, a large button industry grew along the Mississippi River from the 1890s, based on freshwater mussel shell. Muscatine, Iowa, became known as a center of this trade. Workers cut round blanks from shell with tube saws, then shaped and drilled them. Overharvesting depleted mussel beds, and plastic buttons replaced most shell buttons by the mid-20th century. Shell buttons are still made for high-quality shirts and clothing.
Watch dials and jewelry
Thin slices of mother-of-pearl are used for watch dials, where the natural pattern means no two dials look the same. White, gray, black and abalone shell are all used, and some dials are dyed. The slices must be thin, so they are often backed with a metal plate for strength.
In jewelry, mother-of-pearl appears as beads, pendants, cabochons, inlay in rings and earrings, and carved pieces. It is also used as a backing in some doublets and as a base for imitation pearls. Shell beads coated with pearl-like material, often called shell pearls, are common and should not be confused with cultured pearls.
Treatments
Mother-of-pearl is often dyed to produce colors such as pink, blue or black. Some is bleached to lighten it. Thin pieces may be laminated or backed for strength. Reconstituted shell, made from ground shell and resin, is used in less costly items. These treatments should be disclosed, and the page on gemstone treatments covers disclosure norms.
Care
Mother-of-pearl is soft and sensitive to acids, heat and drying. Perfume, vinegar, citrus juice and some cleaners can dull or etch its surface. Wipe it with a soft, damp cloth, and avoid ultrasonic and steam cleaners and long soaking. Store it away from harder gems that can scratch it. The guide to cleaning jewelry safely covers shell, pearl and other organic gems.
Frequently asked questions
Is mother-of-pearl the same as pearl?
They are made of the same nacre. Mother-of-pearl lines the inside of a shell, while a pearl forms as a separate object inside the animal.
What is paua?
Paua is the Maori name for the New Zealand abalone, Haliotis iris. Its shell has strong blue, green and violet colors.
Why does mother-of-pearl shimmer?
Its nacre is built from thin stacked layers of aragonite. Light reflects from many layers and the reflections interfere, producing iridescence.
Is mother-of-pearl durable?
It is soft, about 3.5 on the Mohs scale, but fairly tough because of its layered structure. It scratches easily and is sensitive to acids.