Pearls are judged based on multiple features.These include the thickness of the layers of nacre, shape, body color, overtone color, polish and surface blemishes. Judging a necklace or bracelet, each of these qualities is examined and rated by how well they match going up and down the entire length of the piece. This is called “matching.” Finally are the pearls properly and evenly drilled, and is the necklace strung with professionally tied knots.
Judging pearl quality is complicated and requires years of patient and continuous practice. Quality features apply to a single pearl, pearls on a necklace as well as pearls in all kinds of jewelry.
The thickness of nacre surrounding the center nucleus is the most important determinant of cost of a cultured pearl necklace. Thicker nacre will be produced if the nucleus is left longer inside the mollusk. More layers of nacre produce brighter and richer luster and therefore more beautiful pearls. Luster is defined as the quantity and quality of light that refracts from the layers of nacre and combines with the reflection of light from the surface of the polished pearl. Before World War II, it was not uncommon for pearls to be left growing from 4 to 6 years, producing almost 1mm of nacre around the nucleus, resulting in gorgeous luster. Modern techniques allow farmers to reduce the growth period to 2 years, and the finest quality necklaces today exhibit about .6mm to occasionally .8mm of nacre, which is still more than sufficient to produce wonderful pearls.
Nacre thickness is impossible to measure but can be determined by X-ray. Expert pearl buyers can determine the growth period (and therefore estimate nacre thickness) just by eyeing the luster, and can also easily distinguish the difference between the glow from heavy nacre and the glow from reflected light from surface polish.
Sadly, most pearls are removed and brought to market more quickly. These have less nacre and less luster. Modern polishing techniques have improved surface reflection of light, so when the necklace is new, the pearls still look pretty. This is NOT refracted light from thick nacre. It is merely reflected light from surface polish, and it does not last. Within a few years, many of the pearls become lifeless. It happens gradually, and the process is hastened by the use of perfume and hair spray.
Pearls are cultured in many shapes, by simply inserting nuclei with different shapes. Generally pearls are placed into 3 categories for shape: round, symmetrical and baroque. The best pearls are round, and when all pearls on a necklace form perfect circles, the cost increases significantly. Symmetrical pearls would include oval, button and drop. These are not considered baroque meaning misshapen pearls; rather they are considered fancy shapes, very much like fancy shape diamonds (diamonds other than the round brilliant). Pearls from different parts of the world are classified by different shapes. The dark Tahitian cultured pearls are subdivided into 5 shapes: round, drop, button, baroque and circled. A baroque cultured pearl is misshapen. An out-of-round pearl is just slightly misshapen, but not nearly enough to be labeled baroque.
The most diverse selection of shapes comes from the huge annual production of Chinese freshwater cultured pearls. Almost all these necklaces are inexpensive fashion. However China has continued its experiments with the Japanese Akoya mollusk in hopes of some day producing round pearls with the same color and luster as their Japanese counterparts. As of 2016, China is coming close as there are a few strands now and then with very round pearls, but not yet with the same color and luster.
Gemologists look at the body color of the pearl, which is the color everyone sees. Japanese akoya cultured pearls have 3 body colors: rose, white and cream. We identify that color. We identify how even or uneven it displays fully around the pearl. We identify its intensity and vividness. We then look at what is called Overtone Color(s). Overtone color is iridescence, one of the “light phenomena” of the gem kingdom. An iridescent color is a SECONDARY color appearing to float in a circle above the body color of the pearl. Japanese pearls show pink, blue and green iridescent colors. The rarest and highest quality would be a combination of pink and blue, creating a purple overtone color floating above a white pearl.
Pearls from Lake Biwa Japan grow with many fancy colors. In the 1990’s, strong pink and orange pearls from Lake Kasumingaura reached western markets under the name Kasuminga. These pearls were seldom round and exhibited wrinkles and knots along their surfaces. Black, brown and reddish black pearls from Japan are dyed, and we now know that most of the gray and black Tahitian cultured pearls are treated to deepen or make their natural color more even. Certain mussels used to culture freshwater pearls in China produce intense mauve, orange, purple and pink colors in addition to white. These fancy colored pearls may be stabilized by a bleaching process before they are strung into necklaces.
Like most colored gemstones, most pearls are enhanced in some fashion after they are removed from the mollusk. The question of what kind of treatment is ok and what kind is deceptive continues to be debated within the industry. I believe that all treatments to improve the quality of a pearl should be disclosed to the buyer. Most pearls are bleached and then polished. Bleaching is known to create a uniform color. After bleaching, most Japanese Akoya pearls are then colored to produce light pink overtone. Such pearls are not considered dyed, and the pearl industry has accepted this process for decades, without the need to disclose it to the buyer. Dark pearls from Japan are created by immersing them in a silver nitrate solution. Certain large pearls from Tahiti showing a dark color are also treated in this fashion.
Blue, green, purple and brown aniline dyes are also used to color cultured pearls. Gemologists can sometimes test these pearls for dye by using ultraviolet fluorescence and or by microscopic examination of the drill hole, and there are additional tests available at advanced gemological laboratories.
A difficult challenge is determining the origin of golden color in South Sea cultured pearls. Chemically dyed pearls are substantially less expensive than natural golden South Sea pearls. The GIA is currently researching a reliable test for these pearls even though the pearl industry admits that the great majority of golden pearls are dyed in some fashion.
Some Akoya cultured pearls and some white and black South Sea and Tahitian pearls are coated with a type of silicon polymer film to improve luster. There are several methods of testing for this type of enhancement.
Few pearls have unblemished surfaces. Gemologists consider the size, distribution, type and visibility of surface markings. Like imperfections in diamonds and inclusions in colored gemstones, pearls grow with many kinds of surface marks. These include bumps, roiled skin, knots, grooves, pits, dull spots, scaly spots, growth rings and many more. If the surface markings are immediately visible, the pearl is downgraded and much less expensive. If the beads must be examined closely to see blemishes, we would call these slight growth characteristics and they would not degrade the value of the necklace very much. Many Chinese freshwater pearls show wrinkling on their surfaces. They are often compared to raisins or prunes.
Especially damaging on any type of pearl are tiny holes that appear in the nacre and continue to the nucleus. Such pearls are inexpensive, even with strong nacre and high luster.
Every pearl on a necklace is polished once it is removed from the mollusk. How carefully and evenly this is accomplished is part of pearl quality analysis. As stated earlier, even pearls created with very short growth periods, with minimal nacre, can be nicely polished, though the polish will fade over the years as there is no nacre to support it underneath.
The best cultured pearl necklaces consist of pearls which match one another in each and all of the quality characteristics and size. With any of the quality features, if they are inconsistent up and down the strand, the cost drops quickly. If almost all pearls in a necklace are round and only a few are slightly out-of-round, the cost is downgraded only slightly. Matching applies to the very finest Japanese akoya cultured pearls, the finest South Sea cultured pearls and the most inexpensive cultured pearls, regardless of where they are grown. No more than 25% of the value of a pearl necklace can be attributed to matching.
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