Size

Pearl measurements are stated in millimeters, rounded to the nearest 0.5 mm. Trade members measure the diameter of drilled individual spherical pearls perpendicular to the drill hole and state that measurement as its size. For individual pearls of any other shape, measurements consist of length and width, and sometimes depth.

A uniform strand consists of pearls that are nearly the same size and usually spherical. Trade practice is to measure several pearls in the strand and state the narrow size range in half-millimeter increments. For example, a strand might be called an “8 to 8.5 mm strand.”

Some strands are graduated strands, with the largest pearl in the center and progressively smaller pearls approaching the clasp. Measuring the diameters perpendicular to the drill hole, the sizes are stated as a range—for example, 3 to 7 mm, which might also be expressed as 7 to 3 mm.

Generally speaking, when other value factors are equal within a pearl type, the larger the pearl, the greater its value. Each pearl type has a size range that includes the common sizes for most pearls of that type. There might be a relatively small number outside the range on both ends, but that volume is not commercially significant.

For example, most spherical akoya cultured pearls—often just called “akoyas”—are between 2 and 11 mm in diameter. When other value factors are equal, larger akoyas are more valuable than smaller akoyas. However, you can’t compare the values of a spherical akoya and a spherical South Sea cultured pearl of the same size because South Sea pearls are generally much larger. For example, South Sea cultured pearls measuring 11 mm in diameter are common, while 11-mm akoyas are rare. The akoya’s rarity makes it much more valuable.

Valerie Power/GIA, courtesy Robert Wan
A millimeter gauge is the ideal tool for measuring cultured pearls.