The size of the pearl is measured by its diameter in millimetres.
The Larger the Pearl Size, the Greater the Likely Value
Generally speaking, the larger the size of the pearl, the greater its value. Larger pearls not only require larger mussels to produce them, but they require much more time to develop inside the mussel. In addition, because it is difficult to maintain a perfectly spherical shape while the pearl develops, large round pearls are extremely rare and valuable.
Freshwater Pearl Sizes are Often Measured in Full Millimetres
Freshwater pearls are more often given a full millimetre range of size. For example, a strand of 8 mm freshwater pearls may be measured 7-8 mm in size. Freshwater pearls range in size from as small as 3mm to as large as 10mm. Pearls below 7mm are considered small in size. 7mm until 8mm are generally the more popular sizes selected for necklaces, while anything above 8.0mm is considered to be more rare and valuable, especially in the higher qualities.
Pearl Sizes will Vary in Strands
When shopping for pearls it is important to note that strands are rarely of uniform size. The internationally recognised standard for measuring pearls will always have a .5 mm differential within an individual strand. As per industry standards today, a strand measuring 7.5 mm will actually be composed of pearls measuring 7-7.5 mm. This should be noted in any appraisal or description of the pearls. The smaller pearls will be located near the clasp, and the larger pearls will be in the centre of the necklace. This is considered a uniform strand, not a graduated strand.