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1.) Nacre thickness and quality is the most important factor in terms of beauty and with cultured pearls, in knowing how long they will last. Nacre thickness determines the life span of the pearl and the quality determines how the light travels through the layers. Sometimes pearls with nacre fail to exhibit the intensity of luster or orient that is expected because the nacre layers didn't crystallize properly. The crystals may lack transparency, the layers may not be uniform, or the layers may not be properly aligned. The result is a pearl with lower luster. Yet, it takes a thick layer of nacre to produce high luster, so always look for a pearl with such a luster to guarantee lasting beauty.
2.) Luster and orient which is the sharpness and intensity of the images reflected from the pearl's surface (luster) and the underlying iridescent play of colors (orient) that distinguish the pearl from all other gems. The higher the luster and orient, the finer the pearl. When judging luster and orient, look at the shadow area of the pearl, not the shiny, reflective area. Don't confuse "shine" with the deep iridescent glow created by the combination of luster and orient.
3.) Color consists of body color and overtone. The body color is the basic color: white, yellow, or black. The overtone is a secondary color (its tint), such as the pinkish overtone in fine white pearls. The rarest color would be the white pearl with rose-colored overtones and the most expensive. The creamier the color becomes, the less costly they become. However, today the rose tint is often imparted to the pearl through artificial means. If you use a loupe to examine the drill hole, you may be able to detect the color enhancement by a pinkish line between the nucleus and nacre. Cultured pearls are available in many colors which often are caused by surface dyes or irradiation techniques. Black pearls may need to be sent to a gem-testing labratory to be tested in order to tell whether they have been treated or not. Some colored pearls are easily detected and some may need a gemologist to determine whether they are enhanced in color.
4.) Cleanliness refers to the pearl's freedom from such surface blemishes as small blisters, pimples, spots, or cracks. Occasional small blemishes are not uncommon, however, if numerous they are unsightly and less durable. The cleaner the skin, the better. Keep in mind, that thin nacre will often have a clean surface.
5.) Shape in pearls is divided into three categories: spherical (round), symmetrical (button and pear-shaped), and baroque (irregular shaped). Round, naturally being the most expensive according to the perfect roundness of the pearl. Symmetrical pearls are judged on evenness and good symmetry and are less expensive than round, yet more expensive than baroque. As with cleanliness, thin nacre pearls are very often round due to the nucleus being round.
6.) Size naturally is a factor in the cost of a pearl. Pearls are sold by weight (grains) and 4 grains equals 1 carat. Cultured pearls are sold by millimeter size in which round will have one number and other shapes will have 2 numbers for the length and width of the pearl. 2 millimeter pearls are considered very small, while those over 9 millimeters are very large. Large cultured pearls are rarer and more expensive and the jump in cost when they are over 7.5mm is dramatic. The price will increase rapidly every 0.5mm from a pearl that is 8mm and up.