One of the most important items to consider when purchasing pearls is the grade of the pearls. As you would not buy a diamond without understanding its attributes (size, clarity, inclusions, color, etc.), you should not buy pearls without first understanding their attributes.
The core attributes to consider with pearls are: type, size, shape, luster, inclusions, nacre and matching. All of these combined determine the value and quality of any piece of pearl jewelry. As there are no internationally agreed upon grading scales for pearls (unlike most other gems), you must pay close attention to the attributes and not just the scale provided by the vendor. Although most pearl retailers adopt a GIA-based scale (A-AAA), some exaggerate this scale (A-AAAAA) and some are less strict on what is classified as a top level pearl. That is why you need to look closely at the attributes to understand what you are buying. The table below outlines each attribute:
Type
The type of pearl significantly impacts its value. All things being equal, a Freshwater pearl would be the least expensive, next would be an Akoya pearl, next would be a Tahitian pearl, and finally a White or Golden South Sea pearl. Of course, this is only with all other things being equal (Size, Shape, Luster, Inclusions, and Nacre).
Size
The larger the size of the pearl impacts the greater the value, sometimes exponentially as you reach the top sizes.
Shape
The more close to round a pearl is, typically the greater its value. So most expensive would be round, next would be off-round, next would be oval or drop, and eventually this would end with the least expensive shape being baroque (no symmetry on either axis).
Luster
Luster, or iridescence, is the brightness or radiance of light from a pearl. The greater the luster, the greater the value of a pearl.
Inclusions
Inclusions, or blemishes, are imperfections in the surface of the pearl. The fewer inclusions, the greater the value of a pearl.
Nacre
Nacre is the layers of calcium carbonate and conchiolin that form over the bead (or irritant). Nacre is important because it determines the longevity of a pearl. A pearl with a thin nacre will not last as long as a pearl with a thick nacre.
Matching
Matching examines a set of pearls as a whole - specifically how well they resemble or harmonize with each other.
