Generally, the larger the size, other factors being equal, the higher the value. However price jumps between pearl sizes are often uneven. As the size reach the 6-mm or above, pearl prices tend to increase much higher. Different pearl-types are measured by different methods with different minimum units' expression.
Larger pearls not only require larger oysters to produce them, they also require much more time to develop inside the oyster. An "average" pearl can take 2-3 years to develop, significantly larger pearls can take twice as long. In addition, because it is difficult to maintain a spherical shape while the pearl develops, large round pearls are extremely rare and valuable.
The Importance of Size
Cultured pearl types typically have their own specific size range. However, culturing practices are changing, and you will find more pearl sizes among the different types.
Traditionally, Akoya is on average and max the smaller pearl type, via Freshwater pearls and Tahitian to the considered larger South Sea pearls. The value of the size really depends on how unusual it is. Large Akoya cultured pearls are not large by Tahitian standards but a large one would be of higher value than a Tahitian pearl of the same size.
Factors Affecting Size
- The size and vitality of the mollusk that produced it,
- the size of the nucleus. The nucleus is the irritant, such as a bead in cultured pearls, at the heart of the pearl,
- the time the mollusk spends underwater before the pearl was harvested.
Freshwater Pearl Sizes
Cultured Freshwater pearls come in sizes that start as small as 4-mm and range up to 12-mm and lately, larger! The most common and popular Freshwater pearl sizes found today range from 6mm up through 10mm.
Akoya Pearl Sizes
Japanese Akoya pearls range in size from 2mm "seed" pearls up through 10-mm. The most popular and common sizes you'll find today range from 6-mm through 7-mm pearls. Akoya pearls greater than 8 millimeters are considered extremely rare.
South Sea Pearl Sizes
Both White and Golden South Sea pearls are famous for being the largest pearls available in the world. They range from 8mm up through 16-mm on average, and can sometimes attain sizes of 20-mm! The most popular pearls range in size from 9-mm through 14-mm.
Tahitian Pearl Sizes
Exotic black Tahitian pearls are naturally larger-sized pearls, ranging from 8-mm up to 16-mm and larger. The most common and popular size range is from 9-mm up through 12-mm.
Pearl size measuring methods
The diameter of every single pearl is measured in millimeters, varying from 2 to 20mm and rounded off to the nearest 0.25-0.50-mm depending on type and range. Round shape: average diameter, fancy shape: maximum diameter, maximum length.
Pearl Sizes in Strands
Pearls in strands are measured differently. Measurements are taken perpendicular to the drill holes and expressed in ranges. If the differential in the strand is greater then 0.5-mm for akoya or 1-mm for other types, the strand is considered graduated, then the maximum diameter of the largest and the smallest pearl size on the strand is stated.
Strands are rarely of uniform size. Per industry standards today, an individual akoya strand measuring 7.5 mm will actually be composed of pearls measuring 7-7.5 mm, with the smaller pearls being placed near the clasp. A strand of 8 mm freshwater pearls may be measured 7-8 mm in size as more often given a full millimeter range and still considered a uniform strand. This is not an absolute standard, as occasional high-quality freshwater pearls are matched similar to akoya pearls with the mere 0.5 mm differential.
Tahitian and South Sea pearls are typically measured individually, but to be considered a uniform strand there may still be up to a 1 mm differential from the clasp to the center. For example, a 9-10 mm strand of Tahitian pearls is considered a uniform strand, but an 8-10 mm strand is considered slightly graduated.