What Size Pearls Should You Choose?

Size is one of the seven value factors used to judge natural pearls and cultured pearls,² but what counts as a large pearl depends on the type.

How pearls are measured#

Pearl sizes are diameters in millimeters. CIBJO’s standards say measurements must be given accurately in millimeters, and any rounding must be downward, never up.¹

A single round pearl is measured across its widest diameter, or, if it is drilled, across its widest diameter at right angles to the drill hole. Pearls of other shapes are described by their maximum length and their smallest and largest diameters.¹

For a strand, the description gives the largest diameter of the largest pearl and the smallest diameter of the smallest, whether the pearls are all about the same size or graduated.¹ On a report from SSEF, the Swiss gemological laboratory, a graduated necklace gets diameters for the two smallest pearls, which are often the ones next to the clasp, two in between and the largest, and a necklace of pearls of about the same size also gets five diameters.⁵

Natural pearls are also weighed. SSEF gives the weight of natural pearls in carats and in grains, where 1 grain is 0.25 carat, and the weight of cultured pearls in carats only.⁵ In the trade, CIBJO notes, cultured pearls may also be weighed in momme, kan, liang, carats or grams.¹

Text version of this diagram

Eight circles in two rows, all drawn to one scale and each labeled with its diameter. The top row shows the four smallest sizes and the bottom row the four largest. Each circle is two millimeters wider than the one before it, so the largest is eight times as wide as the smallest.

Typical sizes by type#

Each type grows within its own range. The table gives the typical sizes and the full range for each main type in the Learn About Pearls dataset. GIA cautions that pearls of every type can occur outside their typical ranges.⁶

• Akoya are typically 6–8 mm, and Akoya over 10 mm are rare.^{7 8} In the Japanese Akoya classification in CIBJO’s guide, pearls of 3 to 4 mm are called rin-dama, or extra small, 5 mm pearls are small, 6 to 7 mm pearls are medium, and pearls of 8 mm or more are large.⁴
• Freshwater pearls cover the widest range, typically 3–15 mm.⁴ Most are small, and the larger ones are often bead-cultured.⁶
• Tahitian pearls are typically 8–14 mm, though CIBJO’s guide gives a wider typical range that starts at 4 mm.^{4 6}
• South Sea pearls are the largest cultured pearls,³ typically 9–16 mm, with rare pearls over 20 mm.⁴
• Keshi, which form without a bead, run from under 1 mm to 16 mm (larger ones are very rare).^{1 4}

The difference starts with the animal. GIA notes that the oyster that grows South Sea pearls is larger than the one that grows Japanese saltwater pearls, which gives it the potential for larger pearls.²

How size affects value#

When the other value factors are equal, larger pearls are rarer and more valuable than smaller pearls of the same type.² GIA counts size among the reasons South Sea pearls are valued, along with their thick nacre, their long growth and the limited conditions they need.¹¹

Size is only one of the factors, though. GIA calls luster perhaps the most important, and says numerous or severe surface blemishes can severely depress a pearl’s value.² The page pearl quality explains each factor.

Choosing a size#

Decide on the type first, because a size that is large for one type can be small for another. An 8 mm pearl counts as large in the Japanese Akoya classification, but it sits at the small end of the South Sea range.^{4 6} To see how big a size looks, the pearl size visualizer draws a pearl of the size you choose and can be set to life size on your screen.

For a necklace, decide too whether the pearls should be all about the same size or graduated. In a graduated strand the pearls change in size along the strand, and the smallest are often the ones next to the clasp.⁵ The page necklace lengths covers how long a strand should be, and what pearls are worth covers price.