As the popular saying goes, those pearls with a diameter less that 7mm are just beads, while those with more than 8mm are the right precious treasures. 7mm to 9mm pearls are the most popular ones, it is hard to abtain 10mm-pearl, not to mention those South Sea pearl and the Black pearl with a 11mm diameter.
It is hard and unscientific to define a pearl's diameter just by our eyes. The precise figuure should be measured by a professional measure tool -- Disc Vernier caliper. Measure three times and write down the average maximum diameter and the average minimum diameter.
There are different ways to show the diameter of the different pearls. Use the minimum diameter for those perfectly round, round and off-round pearls, and both maximum and minimum diameter for other shape pearls, like oval, drop and baroque pearls. E.g. A round pearl's diameter is 6.0mm, and a oval pearl's is 6.0 - 7.5mm.
