Luster is a very important aspect in determining the value of a pearl. The better the shine and reflection qualities a pearl has, the better grade it belongs to. So always check the shine of the pearl carefully.
It is rare to find a pearl that is perfectly round. The rounder the pearl, the more valuable it is. However, there are some natural varieties that are quite valuable but are more inclined towards a linear shape rather than a round shape. Study the different types to understand the grading on shapes.
Any pearl with a surface that is free from any blemishes belongs to a higher grade. Generally the grading is done in AAA, AA and A quality pearls, based on the surface. The highest quality AAA has a 95% blemish free surface, the AA quality has at least a 75% blemish free surface and the A has at least a 25% blemish free surface. Lower than A qualities are deemed unfit for jewelry.
Larger sizes are graded higher than the smaller sizes. The general size starts from 8mm and can go up to the rare occurrence of 18mm. It is measured across its diameter.
Natural or Cultured
The value of a natural pearl is a lot more than the value of a pearl which is cultured or simply prepared through the farming process. It is important to know whether the pearl you are looking at has come from a farm or is natural.
A higher weight carries greater value, though other factors hold greater importance than this. It can be measured in carats or momme. Not all jewelers add the weight but it is now something that is being adopted more frequently, to make the purchasing process easier for the customers.