Most of the time, pearls are described primarily in terms of their size (diameter) expressed in millimeters, rather than their weight. However, the weight of a pearl can also be provided, especially if the pearl is particularly large.

Pearl weights are generally expressed in either carats, grains, or momme . Typically cultured pearl weights are expressed in carats or momme, while natural pearl weights are expressed in grains. However, the three types of units can be converted into one another, and also converted into grams for easier understanding.

One carat = 4 grains = 200 milligrams = 1/5 gram

One grain = 1/4 carat = 50 milligrams = 1/20 gram

One momme = 18.75 carat = 3750 milligrams = 3.75 grams

When pearls are purchased wholesale they are often purchased by weight in lieu 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 ever recorded was originally called the "Pearl of Allah," but is now known as the "Pearl of Lao-tze." It was discovered by a pearl diver in 1934 near the island of Palawan in the Philippines. This pearl is 238 millimeters in diameter (over 9 inches!) and weighs in at 6.4 kilograms! That's 6400 grams (over 14 pounds), or 1,280 carats!

To put things into perspective, a 7.5 millimeter cultured pearl will weigh about 3 carats, or 3/5 gram.

The Pearl of Lao-tze is estimated to be worth about $40,000,000.