The quality and therefore, the value of a pearl is measured according to a combination of several different factors: the type of pearl, the thickness of its nacre, its lustre, the cleanliness and texture of its surface, its shape, its colour, and its size.
Because the pearl is a naturally occurring organic gemstone created by living creatures, these factors can and do vary widely. Although all of them affect the ultimate value of a given pearl, some of the factors are more objective, while others are more a matter of taste or reference.
The type of pearl is perhaps the most basic factor to consider: whether the pearl is a freshwater pearl, an Akoya pearl, a South Sea pearl or some other variety. Each type of pearl is created by a different species of oyster, usually living in a different region of the world and under varying climatic conditions -- all of which have an impact on the characteristics of the resulting pearl.
Some types of pearls are much rarer than others, for example, increasing their value significantly for this reason alone. Other types are more common, meaning that the pearl's ultimate desirability and value will be determined by factors other than the type. South Sea and Tahitian Pearls are generally larger than Akoyas and each type tends to have its own distinctive range of colours.
Nacre is the substance from which the pearl is actually created. Pearl characteristics such as colour and lustre are actually characteristics of the nacre itself. In general, the thicker the nacre, the more valuable the pearl. A pearl's lustre is a measure of its brilliance and reflectivity. High-quality pearls are bright and shiny -- you should be able to see your reflection in them. Lower-quality pearls have a more chalky or dull appearance. In general, saltwater pearls have a greater lustre than freshwater pearls.
The appearance of the surface of the pearl is perhaps one of its most critical characteristics. The surface should be smooth and clean, without bumps, spots, discolourations or other disfiguring characteristics. As noted, it should be shiny and reflective, rather than dull and chalky.
The shape of the pearl is one important area where "value" and personal taste may diverge. Perfectly round pearls (Perlen) are extremely rare and therefore very expensive. However, pearls come in a wide variety of interesting and unique shapes and you may find some of these shapes even more appropriate to your own taste and personality. Button pearls are slightly flattened into a disc-like "button" shape; drop pearls are teardrop-shaped; ringed pearls exhibit a series of concentric indentations or rings; and baroque pearls are abstract and asymmetrical.
Many high-quality pearls can be found in these alternate shapes -- often at lower prices than a perfectly round pearl of the same quality -- and you may find the unique shape to be even more personally appealing and satisfying.
Pearl colour is another area where the most "valuable" pearl may or may not be the most appropriate pearl for you as an individual. Pearl colours range almost the entire spectrum from white to black. Some naturally occurring colours include silver, cream, champagne (gold), green, and blue. Note that the pearl's overtones are distinct from its basic colour and they allow pearls in the same colour category to have much-different looks and hues. Although some colours are naturally rarer than others and therefore more expensive, colour is another factor where all other things being equal, the "best" colour for you is largely a matter of personal taste and what looks good on you as an individual.
Finally the size of the pearl has a direct bearing on its quality and price. Larger pearls command higher prices (again, with all other factors being equal).The size of the pearl is measured by its diameter in millimetres. Akoya pearls tend to be in the 6-8 millimetre range, while South Sea, Tahitian and freshwater pearls tend to be somewhat larger. By contrast tiny "seed pearls" can be 1 millimetre or less in diameter. Most pearls sold today tend to fall into the 6.5-7.5 millimetre range.