Pearl size is measured in millimeters by their diameter. Although pearls can be found less than 5 mm in size, these are generally considered too small and will be left in the oyster to reach a larger size.
Although generally smaller than other ocean varieties, Akoya are the most renound pearls in the world. They generally come in size anywhere from 6.5 to 9.5 mm, although 7.5 is average.
The black Tahitian pearls are the largest pearls measuring as much as 25 mm (1 inch in diameter)! A pearl’s value increases sharply with its size as long as it maintains other aspects of its quality like shape and color. Tahitian black pearls are extremely rare and very valuable and are known for being the only natural black pearl available in the world. They usually range in size from 9mm to 16mm. They are harvested in French Polynesia from the Pinctada Margaratifera oyster.
When looking for a large lighter colored pearl the South Sea is exceptional and extremely rare with an average size of 13.0mm, which is due to the large size of the pinctada maxima oyster that they are found in.
China has created a revolution in the pearl industry by pefecting a methodology of harvesting cultured freshwater pearls that rival the finest quality of Akoya pearls, and China, since the 1990's, has been producing round white pearls of up to 7mm for a fraction of the cost of Japanese cultured saltwater pearls.
An average pearl can take anywhere from 2 to 3 years to develop, but much larger pearls can take up to 6 to 7 years and require larger oysters. Large pearls that keep a good shape are extremely rare and valuable. Also remember that in terms of the value of the necklace as a whole the necklace that has very uniform sized pearls are worth much more.
The internationally recognized standard for pearl necklaces is to range no more than a .5mm difference between different pearls on a strand. So a 7.5mm strand of pearls on a necklace will actually range from 7mm to 7.5mm with the smaller pearls being located near the clasp in the back of the necklace.
Freshwater pearls are given a wider range of size because of their lack of uniformity in shape and can vary a whole 1mm instead of .5mm. Necklaces that are not uniform are called graduated and this is commonly done with larger pearl necklaces, especially in black Tahitian pearl necklaces, which can vary by 2mm.
Ranges:
6.5-7.0 mm
7.0-7.5 mm
8.0 to 8.5 mm
9.0 to 10.0 mm
10.0 mm and above