Pearls are graded using 5 different categories.
The first of these is the SIZE. The Japanese pearls are grown from around 2½ mm up to 9½ mm. The South Sea pearls grow from 8½ mm up to 20 and sometimes 23mm in size. Mabe pearls range in size from 8mm up to 18mm, and the Fresh water pearls grow from 3mm up to now 9–9 ½ mm.
The second category is LUSTRE. Depending on how healthy the oysters is and the quality of the surrounding environment i.e. food in the water etc., depends on how well the nacre is secreted and layered on the pearl. If the conditions are right the pearl surface is very even and reflects the light and diffracts some of the light as it penetrates the surface of the pearl and is diffracted at different angles. So you see different colours of the rainbow know as the ‘Orient of the pearl’. The surface, if smooth reflects an almost mirror finish. So a pearl can be graded as having a high lustre down to a chalky lustre.
The third category is the SURFACE BLEMISHES. Being produced by nature, the surface of a pearl is not always even and smooth and may have some pits and bumps on the surface. A pearl can then be graded as having a clean surface to a heavily pitted surface.
Some pearls are graded using an ‘A’ to ‘D’ grading (A pearls are flawless, B have up to 5 blemishes and 75% of their surface clean, C pearls still have 5 blemishes with 50% of their surface marked, whilst D pearls have more than 50% of their surface marked).