Normally, pearls are described primarily in terms of their size (diameter) in millimeters, rather than their weight. However, when pearls are purchased wholesale on farm before precision machining, they are often purchased by weight. And if the pearl is particularly large, the weight of a pearl can also be provided. Pearl weights are generally expressed in 3 unites: carats, grains, or momme. Typically cultured pearl weights are measured in carats or momme, while natural pearl weights are expressed in grains.
The size of natural pearls is often expressed in pearl grains. One grain equals 0.25 carat. Natural American freshwater pearls may be sold according to their carat weight. The gram is commonly used to express the weight of cultured freshwater pearls, although carat weight is also used. One carat = 115 gram. Or in other words 5 carats = 1 gram. Weight equivalences are summarized in Table 7.1. The approximate weight of individual loose pearls can be calculated by referring to both Tables 7.1 and 7.2.
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
Table 7.2 (Based on data from the Shima Pearl Co.)
Size Pieces Per Momme Size Pieces Per Momme
2.5 mm 160 pcs 6.5 mm 9.3 pcs
3 mm 90 PCs 7 mm 7 pcs
3.5 mm 63 pcs 7.5 mm 6 pcs
4 mm 40 pcs 8 mm 5 pcs
4.5 mm 27 pcs 8.5 mm 4.2 pcs
5 mm 19 PCs 9 mm 3.5 pcs
5.5 mm 15 pcs 9.5 mm 3 pcs
6 mm 12 pcs 10 mm 2.5 pcs
To make things easier to understand or imagine, a 7.5 millimeter cultured pearl will weigh about 3 carats, or 3/5 gram.
When pearls in a large wholesale are often purchased by weight instead of strands. The most common standard of weight used when buying Chinese pearls is the kilo. When shopping for pearls in Japan，momme is standard.
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.