The size of the pearl along with a high lustre and complexion is considered highly valuable.
Australian South Sea Pearls are the biggest pearls in the world. They are cultured by inserting a nucleus into the Mother Shell (Mother of Pearl). It may start it's reproductive life on a pearl farm with a 8mm nucleus which is inserted by a trained technician. This nucleus is made from a shell itself. After two years of careful cultivation the size of the pearl will be around 8mm+. If this particular shell is of a high reproductive standard, it will then be used again for another two years with a slightly bigger nucleus; around 10mm, then so on and so on until the shell may produce a 20mm pearl at it's height of reproduction (although this is a very rare size).