A pearl is a natural concretion formed within the soft tissue of a living shelled mollusk, such as an oyster or mussel. This formation is a biological defense mechanism where the mollusk deposits layers of calcium carbonate, primarily aragonite, around an irritant. Pearl size varies dramatically, determined by whether it formed naturally or through cultivation, the type of host mollusk, and the time spent in the water. Sizes range from tiny seed pearls less than one millimeter in diameter to massive, non-gem quality specimens weighing many pounds.
The Record Holders: Maximum Pearl Size
The largest pearls ever discovered are extreme outliers, often found inside the massive Tridacna gigas, or giant clam. The current record holder for the world’s largest certified natural blister pearl is the “Giga Pearl,” which weighs an extraordinary 27.65 kilograms (61 pounds) and measures 39.37 cm by 22.86 cm. This massive size is possible because the giant clam itself can grow up to 1.4 meters wide and weigh over 250 kilograms.
Before the Giga Pearl was certified, the “Pearl of Lao Tzu,” or “Pearl of Allah,” was considered the largest, weighing 6.4 kilograms and measuring 24 centimeters in diameter. Neither of these clam pearls possesses the iridescent luster of jewelry-grade pearls, as the giant clam does not produce nacre. In contrast, the largest perfectly spherical, nacreous cultured pearl is significantly smaller, with the “Paspaley Pearl” weighing 60.94 carats.
Factors That Determine Pearl Growth
The size of any pearl, whether natural or cultured, is fundamentally limited by the biology of its host mollusk. The largest pearls come from the largest host species, such as the Pinctada maxima oyster used for South Sea pearls, which can grow to over a foot in diameter. Conversely, the Akoya oyster (Pinctada fucata martensii) is the smallest commercial pearl-producing mollusk, restricting the final size of its pearls.
For cultured pearls, the initial size of the implanted nucleus is a direct predictor of the pearl’s final diameter. Pearl farmers insert a spherical bead, typically made from a freshwater mussel shell, and the final pearl will be only slightly larger than this nucleus. The largest nuclei can measure up to 21 millimeters, a size that small oysters cannot physically accommodate.
Cultivation time directly correlates with the thickness of the nacre deposited around the nucleus. South Sea oysters are often left in the water for two to four years, resulting in a thick nacre layer that can measure between 2 and 6 millimeters. This extended growth period contributes to the pearl’s size, whereas Akoya pearls, which have a shorter cultivation time, average a nacre thickness of about 0.5 millimeters.
Size Ranges of Commercial Pearl Types
In the jewelry market, pearl size is measured in millimeters (mm) and varies significantly among the four main types. Akoya pearls are known for being the smallest, with a typical size range of 2.0 mm to 11.0 mm, though anything over 9.5 mm is considered extremely rare. The most popular and versatile size for classic Akoya necklaces is often considered the 7.0 mm to 7.5 mm range.
Freshwater pearls generally occupy a similar size bracket to Akoya, ranging from 2.0 mm up to about 15 mm, with the 7.0 mm to 8.0 mm size being the classic choice for everyday wear. However, newer, bead-nucleated Freshwater varieties, such as Edison pearls, can now regularly reach 15 mm and sometimes larger.
Tahitian and South Sea pearls represent the larger end of the commercial spectrum. Tahitian pearls, often called black pearls, typically range from 8.0 mm to 18 mm, with 9.0 mm to 12.0 mm being the most common sizes found in jewelry. South Sea pearls are the largest commercially harvested type, starting at about 9 mm and commonly reaching 20 mm, with the average size falling in the 13 mm range.