|Surface Purity & Defects|
This attribute is also known as the "skin purity" of the pearl. As it can be expected: the lesser the number of imperfections on the surface of the pearl, the higher its value. A pearl with no visible defects (zero imperfections or flaws) can be considered a Gem (but it must also have nice luster and good orient/chroma).
Sea of Cortez Pearls are only available in the following grades: Gem, A and B. Each different company or farm will have different grading systems -altough most are based upon the same set of grading guidelines- so a company's "B" grade may only be as good as another's "C" grade or quite the opposite: a "B" grade pearl could be another's "Gem Grade". Because of this, you cannot rely on the grading system to make comparissons between different varieties of pearls, and sometimes not even between the same variety.
An "A" grade pearl has 1 to 3 small defects (less than 10% of its area), a "B" grade has anywhere between 3 and 7 small imperfections (10-33% of its area). "C" grade has large and visible spots on most (33-66%) of its surface. Beyond this amount of defects, they can be considered a "D" grade or even "Reject" pearls. It is fairly common for a pearl to have surface defects. After all, a pearl is a living creation, not a cold hard stone dug from the Earth. One should allow for defects in a pearl, just as we allow leather goods to have skin defects, without which they would look just like plastic...