Normally, pearls are described primarily in terms of their size (diameter) in millimeters, rather than their weight. However, when pearl wholesalers buy large lots of cultured pearls on farm, they are often charged according to the weight of the pearls. Also, if the pearl is rarely large, the weight of a pearl can also be provided. Pearl weights are generally expressed in 3 unites: carats, grains, or momme.
The measure widely used is the momme, an ancient Japanese unit of weight which equals 3.75 grams or 18.75 carats. ( 1000 momme equals a Kan-another larger Japanese unit of weight). Typically cultured pearl weights are measured in carats or momme, while natural pearl weights are expressed in grains(One grain equals 0.25 carat.). The gram (1 gram = 5 carats) is commonly used to express the weight of cultured freshwater pearls, although carat weight is also used. Natural American freshwater pearls may be sold according to their carat weight. The most common standard of weight used when buying Chinese pearls is the kilo.
The approximate weight of individual loose pearls can be calculated by the Table 7.1 Pearl Weight Conversions
I carat (ct) = 0.2 g = 0.007 oz av = 4 p grains = 0.053 m
1 gram (g) = 5 cts = 0.035 oz av = 20 p grains = 0.266 m
I ounce avoirdupois (oz av) = 28.3495 g = 141.75 cts = 565 p grains = 7.56 m
I pearl grain (p grain) = 0.05 g = 0.25 ct = 0.013 m = 0.0017 oz av
I momme (m) = 3.75 g = 18.75 cts = 75 p grains = 0.131
To make things easier to understand or imagine, a 7.5 millimeter cultured pearl will weigh about 3 carats, or 3/5 gram. The largest pearl was recorded be discovered in 1934 near the island of Palawan in the Philippines, it is 238 mm in diameter and weighs at 6400 grams (over 14 pounds), or 1,280 carats! And it is estimated to be worth about US$40,000,000.
Other Pearl Quality Factors