Pearl Composition and Structure
Mollusks filter microscopic food from the water they live in. The water is also the source of the chemical elements they use to make their shells, bodies, and organs. One of the organs is the mantle. It’s similar to skin and forms the lining of the mollusk’s shell. Cells on the shell side of the mantle secrete complex liquids that harden to build the shell. One of those hardened liquids is called mother-of-pearl. When mother-of-pearl composes all or part of a pearl, it’s called nacre.
Nacre is made mostly of conchiolin and layers of calcium carbonate platelets. Conchiolin and its related substances are proteins. They control platelet production and also form a matrix that holds the platelets together. Depending on structure, the calcium carbonate platelets might be aragonite or calcite, or a mixture of both.
Aragonite and calcite are both composed of the same elements: calcium, carbon, and oxygen. But they have different crystal structures. This makes a difference in the way they interact with light. This interaction contributes to the quality of a pearl’s appearance in reflected light, known as luster.
Aragonite crystals and light interact to create complex luster, while calcite crystals and light create a simpler shine that’s similar to porcelain.
Another effect of nacre’s extremely thin layers is that white light sometimes travels through it at different speeds and in different directions. This causes light to break up into its component spectral colors, creating an iridescent rainbow effect. This effect is called orient when it’s seen in a pearl or mother-of-pearl. The presence and degree of orientation depend on the pearl type and shape, its nacre's characteristics, and the viewing angle and surroundings.
How is the color of pearls formed?
The color of pearls is related to the type of mother-of-pearl. Different mother-of-pearl oysters secrete "natural pigments" of different colors.
For example, the mother oyster of South Sea gold pearls is the gold lip pearl oyster - Pinctada maxima, which has golden edges so that it can produce golden pearls.
The mother oyster of black pearls is the black lip pearl oyster - Pinctada margaritifera, which is gray-black and has a strong iridescence so that it can produce many color changes.
The mother oyster of freshwater pearls is the freshwater mussels - Hyriopsis cumingii, which can produce pink to purple "carotenoids", thus coloring freshwater pearls.
Tahitian pearl oyster | South Sea pearl oyster | Freshwater pearl oyster
Color
Pearl color can have three characteristics: Bodycolor, Overtone, and Orient.
- Bodycolor - The dominant, overall color of the pearl. Refers to the color of the nacre itself, which is the color produced by the selective absorption of white light. All pearls display bodycolor. It’s mainly caused by natural pigments embedded in the conchiolin. The pigments vary with mollusk species and with individuals within each species. Other factors that can help determine a pearl’s bodycolor include the color of the mantle tissue implant that’s used for pearl culturing, substances in the water, and the nature of the mollusk’s food.
Different bodycolor of Edison pearls
Different bodycolor of White pearlsGenerally defined, bodycolor has three components. Refers to the Munsell color system, which defines color in three dimensions: hue, tone, and saturation (as shown below).
Munsell's three-dimensional color system1). Hue is the basic impression of color; for example, blue, green, or yellow. Hues are divided into cool and warm tones.
Warm tones: yellow-orange-pink-purple-pink, among which yellow-orange is also called "gold".
Cool tones: purple-blue-blue-green-green.
GIA pearl hue circle2). Tone is the color’s lightness or darkness. Pearls come in a wide range of hues. Tones cover the whole range from very light to very dark.
Warm tones have high brightness, while cool tones have low brightness.
The colors of South Sea gold pearls and freshwater pearls are usually in the warm tones and bright, while the colors of Tahitian black pearls and Purple Edison are usually in the cool tones and low brightness, and darker in color.
Different tones of Tahitian pearls3). Saturation is the color’s strength or intensity. Saturation doesn’t usually attain the richness that it does, say, in some flowers or paint colors, but it’s still a factor in pearl-color descriptions. With other quality factors being equal, the richer the saturation, the higher the value.
Different saturation of South Sea gold pearls -
Overtone—A translucent color that appears over a large area of a pearl’s surface.
Some pearls display overtone, a translucent color that appears over the bodycolor. Overtone is always secondary to bodycolor. When present, it’s usually visible over the entire pearl or a large part of it. The most common overtone colors are green, blue, and pink.
Different overtone of Edison pearls
Different hue and different overtone of Edison pearls
Different overtone of white pearls -
Orient—More than one translucent color over the bodycolor, or surface iridescence.
Orient of Baroque pearls
Fewer pearls display orient than overtone, although both effects are caused by the same complex interaction of nacre and light. (It is the effect produced on the pearl surface due to optical effects such as reflection, refraction, and interference of the nacre layer.) Orient occurs over a much smaller area than overtone and most often on non-spherical pearls with irregular surfaces. Orient is surface iridescence that can consist of several colors of the rainbow. As the beads rotate, the color and intensity of the orient will change.
How Pearl Color Affects Value
For hundreds or even thousands of years, “white” was part of the definition of“pearl,” whether natural or cultured. That began to change when cultured pearls were produced in commercially significant quantities in mollusks and areas that had not traditionally been pearl sources. Colors became part of the pearl vocabulary.
A combination of consumer preference, beauty, and rarity created a hierarchy of value within each major pearl type. Those high-value colors for akoyas are silver and white. For Edison pearls, they're white and purple. For Tahitian cultured pearls, they're the colors known as peacock and aubergine, and for South Sea cultured pearls they're silver and golden colors. Overtone and orient usually enhance value as well.
Higher value might also give trade members financial motivation to artificially create or enhance certain colors after harvesting. When other value factors are equal, natural colors are always more valuable than artificially enhanced colors.
