The size of round pearls is expressed in terms of their diameter measured in millimeters. One millimeter is about 1/25 of an inch. The size of non-round pearls can be expressed in terms of their greatest width and length and in some cases depth. The measurements are generally rounded to the nearest half or whole millimeter.
When shopping for pearls, it's important to be aware that the internationally recognized standard for pearl measurement will always have a .5-millimeter difference for pearls within an individual strand. For example, pearl size varies within a strand, of which pearls classified at 7.5 mm will actually be composed of pearls measuring between 7 and 7.5 mm. that is why a few of the pearls might fall slightly above or below the size indicated occasionally. Generally, a 3.5-mm diameter is considered small, and a pearl of 6 or 7 millimeters is large. Pearls can grow up to 20 millimeters in diameter, which is quite spectacular. Even a single millimeter size difference between two pearls is very noticeable.
Larger size doesn't automatically make a pearl more valuable -there are lots of other factors to consider. Obviously, when all other qualities are equal, then the size of pearls will determine which is the more valuable i.e. the larger the pearl, the more it costs.
Larger pearls need large oysters to develop inside, and they need a great deal of time to grow. It takes several years just to grow an average-sized pearl. Large pearls take much more time.
Usually, large pearls are most suitable for middle-aged or accomplished people. Medium-sized pearls are an elegant and sophisticated choice for younger people, while the very small pearls are typically used for clothing and other decorations instead of jewelry.
Pricing often depends on availability & demand. If a bumper harvest comes to season or too many pearls of one size are harvested, their price will go down. Demand can have an important impact on the way size affects price. If there's a high demand for a specific size, its price tends to increase. This explains why occasionally smaller pearls sell for more than bigger ones of the same quality.
Other Pearl Quality Factors