Table of Contents
 South Sea pearl necklace. Image taken from Allé Jewelry |
Pearl Characteristics
Pearls are naturally occurring organic gemstones that are created inside the shells of certain types of mollusks. They are formed by the concentric accumulation of nacre around an irritant inside the mollusks' shell. This nacre is composed of a mixture of the mineral aragonite and an organic horn substance called conchiolin. This concentric layering creates a characteristic luster and play of color and light. This optical effect can also be seen in mother-of-pearl specimens, which are typically the inner layer of nacre lining the inside of a mollusk's shell. Pearls have several characteristic features that their value is derived from. These characteristics are as follows:
Luster:
Pearls have a characteristic luster known as pearlescence. This luster is often described as a "shine with depth" (Matlins, p. 64)or an inner iridescence. Good pearlescence is denoted by a sharp contrast between the bright center of a pearl and the shaded areas surrounding it. This luster is created by the play of light created by the layering of the nacre. In fact, the quality of luster is directly dependant on the thickness of the nacre layer. Thicker nacre produces better luster.Iridescent Orient:
Iridescent orient is a quality of pearl that creates a soft play of colors across the pearl's surface. Like pearlescent luster, the thickness of the nacre layer is responsible for this effect. However, for iridescent orient to be present in a pearl an unusually thick layer of nacre must be present. In addition to thickness, each layer of nacre must be well crystallized and alligned. These factors work together to create this effect by creating a prismatic effect as light passes through the outer layer and is reflected back. It is rare to find round cultured pearls that display orient, and therefore, these types are highly sought after. 
White pearl with pink orient.
Image taken from Amazing Pearls,.Quality:
While nacre thickness plays a large part in the value and beauty of a pearl, nacre quality is of equal importance. The quality of nacre, or the allignment and uniformity of crystallization, will determine how light travels through the nacre layer. We are as of yet unsure why or how good quality nacre is actually formed. One thing that we do know is that nacre formed at a slower rate will have a better quality. If you consider that time also affects the amount of nacre that is deposited around an irritant, i.e. thickness, it should seem obvious that longer growth or cultivation periods should yield superior quality pearls. However, as I will discuss in the Pearl Production section, this is a bit of an oversimplification. For now, just remember that high quality pearls have both rich luster and orient and these two characteristics cannot exist without fine, thick nacre. 
Black Cultured Pearl Cradle Pendant.
Image taken from Allé JewelryColor:
Pearls come in virtually every color. Due to the iridescent qualities of pearls, there is often more than one color to consider. The primary color, or body color, of a pearl is the overall pearl color at the center of focus. A pearl's overtone or tint is the secondary color that is created through the play of light. It is common practice to include both body color and overtone when naming a pearl type by color. For example, a white-rose would indicate a white pearl with rose colored overtones. The next question that arises from this wide variety of colors is,"what causes a pearl's coloration?" We are not entirely certain of the reasons that pearl coloring is so widely varied. There have been many hypotheses involving water temperature and depth as well as location, but these factors have proved inconclusive. The mollusk species has a significant role in color determination, but even this method of sorting is fallible. The two factors that seem to have the greatest effect on pearl color are the organic pigments within the conchiolin layers (these are due in large part, but not exclusively, to the species) and the color of the nacre due to trace elements in the mollusk's environment.Shape:
Pearls are formed in a variety of shapes. These shapes are generally split into three categories: spherical or round, symmetrical, and baroque.Round pearls are generally what people think of when pearls are mentioned. However, perfectly round pearls are probably the most rare shapes that occur in nature. Therefore, round, spherical pearls are highly sought after and are the most valuable. Secondmost valuable are the symmetrical pearls. Symmetrical pearls are represented by teardrop and pear shaped pearls and have, as their name indicates, good symmetry. Baroque pearl are irregular in shape. This represents a wide variety of irregular types. These pearls may have a wrinkled surface appearance, exhibit concentric rings around their diameter, or simply just have an nondescript shape. Occasionally, transition terms such as semi-round or semi-baroque are used to further explain the shape. 
Baroque South Seas Pearl Necklace.
Image taken from Allé Jewelry
Pearl Types
Akoya Pearls:
These are the type of pearl that one envisions when they think of a pearl. That is to say, they are generally white of very light colored, round, lustrous pearls. The finest quality Akoyas were originally produced in Japan. These pearls are highly regarded because they display a very high luster and are more perfectly round than most other pearls. These pearls rarely get larger than 10 mm in diameter.South Sea Pearls:
These pearls are commonly known as the "queen" of cultured pearls. (Matlins, p.50) This is due to their larger than average size. These pearls are also light in color, but vary a bit more into golden tones. Their size is primarily due to the type of oyster that produces them. This species is much larger than the Japanese oyster and produce pearls ranging from 11 to 14 mm and beyond. Because they are larger, South Sea pearls require a longer cultivation period. This, in turn, results in a more irregular shape.Black Pearls:
Black pearls, as their name implies, have a black to gray body color. Though they are technically a variety of South Sea pearl, a special variety of this oyster, the Black-lipped Pearl Oyster, is responsible for this gem. The size of these pearls is normally larger than 8 mm in diameter and occassionally can be found at a size of 20 mm and above. However, the average diameter of black pearls is around 12 mm. Tahiti is the leading producer of these rare, valuable, and beautiful pearls. 
Black Tahitian Pearl Ring.
Image taken from Allé JewelryAbalone Pearls:
Abolone pearls differ a bit from normal pearls produced by saltwater oysters and freshwater mussels. The primary difference of note is that the abalone is a univalve species and not a bivalve. The creation of the pearl, however, is the same: layers of nacre around an object or lining the shell. The nacre used by this mollusk is highly iridescent and displays a vivid array of colors. For this reason, the mother-of-pearl lining of the shell of the abolone is highly valued as well as the pearls it produces. Until fairly recently, most abalone pearls were naurally produced. Now abalone species are being cultured to produce pearls in the marketplace.Conch Pearls:
Conch pearls are created by another univalve species. These type of pearls are technically not pearls at all. These pearls are not created by nacreous method. Rather than the aragonite and conchiolin forming layers around an object, they appear to settle in globs within the shell of the conch. This results in a smooth, semi-round gem with a characteristic flamelike chatoyancy. These are extraordinarily rare and can only be found in approximately one out of every 13,000 conch shells opened.Mabe' Pearls:
Mabe' pearls are created to provide large, attractive, and affordable pearls to the public. They are created by a process of assembly. Initially, a plastic piece of a variety of shapes is implanted on the interior of a mollusks shell. This mollusk, usually an oyster at a cultivation plant, is then monitored while it produces nacre to surround the plastic. After the cultivation period, the pearl is removed and the plastic core is taken out of the back (nacre won't stick to plastic). A hardened epoxy is deposited in the hollow pearl shell and another section of nacre is matched to fit and glued to the back to complete the pearl. Mabe' pearls are created in a wide variety of shapes, from bow ties to Buddha.Blister Pearls:
Blister pearls are pearls that have been formed and remain encased in the inner lining of a mollusk's shell. These types are used in making brooches, earrings, and pendants. These pearl types are usually cultivated in freshwater lakes. Blister pearls are another way of designing pearl or mother-of-pearl specimens into a desired shape. They are more expensive than mabe' pearls but display a tremendous play of colors due to the angular juxtapositions created by the blister.
Pearl Culture and Production
China is the leading producer of both freshwater and seawater pearls. Most Akoya pearls in the range of 6 mm or less come from China. Very few other saltwater pearl types are being cultivated in this area. Freshwater pearls are cultured in a variety of styles, ranging in color and shape. A fairly new product in this market has been the "all-nacre" pearl which has a nacre "seed" implanted as the nucleus.
Australia is the world's largest producer of South Sea pearls. They also produce a local variety of pearl known as "keshi" pearls. These pearls are highly regarded for their thick nacre, and thus, they are cultivated within the oysters for a longer period of time, sometimes up to three years. Colors are generally in the white to cream range, but occassionally black pearls are produced.
Japan cultures the Akoya, a primary seawater pearl. It has been the site for the production of the most lustrous pearl of its kind. The thick nacre layer is again responsible for this, but in this instance it is the increased nacre production rate that effects the layering. Japan has begun to culture black pearls, but they are much smaller than those produced in Tahiti and French Polynesia.
Tahiti is known for its exotic black pearls. Most of these pearls lie in the gunmetal gray range, but nearly all display a rich orient. These pearls can be found in smaller supply in nearly every color and orient from greens to purples. This high quality orient is a by-product of these pearls' extremely thick nacre. In order for proper formation, they must be cultured for a period of two to three years.
The United States was the first country to develop the process of culturing a new type of pearl: the American freshwater cultured pearl. These pearls are produced and marketed in a number of ways. Many baroque pearls, as well as mabe' and blister pearls come from these methods. These pearls have a very rich luster and orient and come in a variety of colors and shapes.
Final Thought
Links and References
- Allé Jewelry. Pearls. WWW URL: http://allejewelry.com/store/default.cfm. Retrieved 5/2003.
- AmazingPearls.com. WWW URL: http://amazingpearls.com/Pearl%20Necklaces.htm. Retrieved 5/2003.
- Landman, Neil H.; Mikkelsen, Paula M.; Bieler, Rudiger; Bronson, Bennet. Pearls: A Natural History. Harry N. Abrams, Inc. in association with The American museum of Natural History, NY, 2001
- Matlin, Antoinette L. The Pearl Book: The Definitive Buying Guide, 2nd Edition; Gemstone Press, Woodstock, Vermont, 1999.
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