Unfortunately, there is no universally used grading system available for pearls. However, only 2 major grading systems which are in use in the USA is AAA-A system and A-D system.
The Hanadama system
Hanadama grade was originally meant for Akoya pearls. Similar system can also be used for freshwater pearls however no Hanadama grade pearl exists for freshwater pearls.
This is considered as the best quality pearl, minimum 95% of its surface will have no defect. They are perfectly round, with mirror-like lustre and having nacre thickness of 0.4mm or more.
With very high lustre, and minimum 75% of its surface will have no defect. Lustre will also be very high, with thick nacre, however not nice as above AAA grade.
A grade is the lowest jewellery-grade pearl with more than 25% surface may have defects. This type of pearl has thin nacre of .25mm or even less.
In this system, pearls are graded on scale A to D, and A is the highest grade. In French Polynesia this system generally is used for grading Tahitian pearls.
Therefore, it is also referred as “Tahitian system.” If you want to have easier shopping experience then at pearlsonly.co.uk, you can utilize this grading system for all kinds of pearls.
This is the highest-quality pearl having very high lustre with only very few defects within 10% maximum of its surface. All these defective areas are then used for marking the location for drilling holes.
Having high or medium lustre. Its surface can have visible defects, but within 30% of the total area.
Medium lustre having surface imperfections within the range of 60% maximum of the total surface area.
D grade pearls can have number of small defects, but none are too deep and it can be spread over maximum 60% of total surface or few deep defects within 60% of total or also combination of both defects of minor/deep type not more than its 60% surface.
In D-grade of pearl, lustre is irrelevant. Most lustrous pearls can also be graded as D, if the surface is blemished as mentioned above.