Pearls are weighed in Japan; the measure of their weight is the momme, and old unit of weight in both China and Japan, and in use today only in Japan. One momme equals 3.65 grams or 18.75 carats. However in most of the world, pearls are sold by their millimeter diameter. Cost increases as millimeter size increases. Japanese Akoya cultured pearl necklaces, the most commonly seen and purchased in the 20th and 21st centuries, begin at about 2.5mm to 3mm and reach 11mm or a bit more. Necklaces presented to young women in high school are usually between 5.5mm and 6.5mm; those presented to college graduates reach 7.5mm. A pearl necklace measuring between 7mm and 8mm represents the classic jewel in an 18 inch length, and it has served as the standard bearer of a woman’s pearl collection, worn continuously decade after decade.

Pearls on a necklace should vary by no more than one half millimeter; therefore a necklace may contain pearls that range from 7.5mm to 8mm, as long as both sizes are represented on the strand. If the pearls vary by more than one half millimeter, the cost drops. An exception would be a necklace of purposefully graduated cultured pearls, often provided as wedding gifts decades ago. Graduated necklaces are beautiful and go in and out of fashion over the years.

As with other gemstones, as size increases, price increases, all other factors being equal. Consumers should always ask to try on at least two sizes, adjacent to one another. Example: 6.5mm to 7mm and 7mm to 7.5mm. The larger pearls create a more expensive necklace, and for some buyers, the half millimeter difference may not be worth the extra money.

Lower quality pearls have smaller price increases as they increase in size. Finer quality pearls have greater price increases as they increase in size. This is because the longer the nucleus is left in the mollusk, the greater the risk of the destruction of the pearl from disease, ocean upheavals and other factors in the under water environment. Perfectly round pearls with multiple years of growth, beautiful color and relatively blemish free skin are very expensive simply because there are very very few necklaces available with those characteristics. Too many things can happen under water and in some years, a majority of the pearl crop is damaged and unsalable when harvested.

The pearl industry generally divides necklaces into 3 lengths; short necklaces called choker, necklaces about 24 inches called matinee and long necklaces termed opera. At various times, fashion dictates length. Over the 20th and into the 21st century, the 18 inch choker remains the most popular necklace.



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