Pearl Grading
Grading the quality of pearls consists of examining the size, shape, surface, color, luster, orient, nacre thickness, and, if appropriate, matching. 
Generally the size range of natural pearls and cultured Japanese Akoya pearls is from 1 to 10 mm. in diameter.  The larger the pearl, the less spherical it tends to be.  Round saltwater cultured pearls over 7 mm. are considered large, and sizes from 15 to 20 mm. are rare unless considering South Seas and Tahitian cultured pearls.  A general rule is that with all other quality factors being equal, the larger the pearl the more valuable it is.
Regarding spherical pearls, the rounder the better.  The Japanese phrase for an “eight-way roller” pearl refers to a pearl that can roll in any direction due to its exceptional roundness.  Pear or drop shape pearls are particularly popular for use in earrings and pendants.  The more equilateral the pear shape, the higher the value.  In the last thirty years, however, baroque (non-spherical) shapes have become quite popular and have risen steadily in exposure and value.
The ideal pearl possesses no surface blemishes or dimples and appears completely smooth.  The smoother the surface, the greater the luster of the pearl.  Most pearls do not have an absolutely smooth surface, however, and those containing blemishes are drilled at the flaw for use in strand necklaces, rings, etc.  Completely smooth pearls are rare and very highly prized.
Pearls come in white, off-white, cream, golden, peach, pink, black, silver, lavender, and blue.  Color preference in pearls can be subjective and is usually matched to the complexion of the person wearing the pearls.  Pearls have the unique ability among gems to give a subtle glow to the skin.  White pearls with rosé overtones are generally preferred by those with light complexions, whereas darker complexions are enhanced by wearing golden or silver pearls.  Black or near-black pearls look good next to any complexion. 
This refers both to the quantity and quality of light reflecting off the surface of a pearl.  Luster is manifested as the reflection of the light source on the center surface of the pearl.  The sharper and more intense the reflected light source, the greater the luster quality and the more valuable the pearl.
This is the phenomena on the surface of pearls that exhibits iridescence.  Orient can be predominantly one color such as green or pink, or consist of a rainbow of colors in weak, moderate, or strong appearances.
Nacre Thickness
This term refers to the iridescent substance (aragonite mixed with conchiolin) produced in layers by a pearl mollusk that surround foreign irritants that  enter the shell.  Nacre thickness more than any other property listed here is responsible for the durability of an individual pearl.  Generally the thicker the nacre layer, either around the nucleus of a cultured pearl or around a hollow or solid area in the middle of a natural pearl, the greater the luster, durability and value of the pearl.
Pairs of pearls that are identical in size, color, shape and quality command a higher price than two single unmatched pearls of equal quality.  Matching groups and whole strands of pearls of equal color, shape and quality can require anywhere from a few months, for the more commercial strands, to many years, for a fine gem-quality strand of matched pearls.  Because pearls are produced from a living organism, it is harder to control the quality of the final product. 
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