As In the case of many other gemstones: the bigger the size of a pearl, the greater its value. Larger sizes are less abundant in the markets, so they are able to fetch a much better price. Once a pearl attains a size of 10 mm it can command a higher price, but there are other breakpoints as well: 12 mm, 14 mm, 18 mm and 20 mm. This means that the price of a pearl may as well double from the size previous to that break point.
Depending on their origin, pearls can have different sizes: bigger species of pearl oyster can produce larger pearls. These are the usual pearl sizes produced on several species of pearl oysters:
Species of Pearl Oyster: Akoya oyster (Pinctada imbricata=fucata)
Countries of culture: Japan, China, Korea, Viet-Nam, Australia and India.
Size range of pearls (diameter in mm): 4.0 - 10.0
Colors present in their pearls: Light colored: white, cream and yellow. Many artificial colors: white, pink, silver, blue and black.
Species of Pearl Oyster: White/Gold-Lipped (Pinctada maxima)
Countries of culture: Australia, Indonesia, Myanmar and the Philippines.
Size range of pearls (diameter in mm): 9.0 - 27.9
Colors present in their pearls: White, Silver and Yellow/Gold. Color "enhancing" and bleaching are routinely performed.
Species of Pearl Oyster: Black-lipped (Pinctada margaritifera)
Countries of culture: French Polynesia, Australia, Fiji and the Cook and Salomon Islands.
Size range of pearls (diameter in mm): 8.0 - 26.7
Colors present in their pearls: Dark/black, Peacock & Gray. Enhancing and bleaching is also common, but Kamoka and Fijiian pearls are said to have natural colors only.
Species of Pearl Oyster: Rainbow-Lipped (Pteria sterna)
Countries of culture: Guaymas, Gulf of California, Mexico.
Size range of pearls (diameter in mm): 8.0 - 12.0. Average size is 9.2 mm.
Colors present in their pearls: Dark/peacock, Silver-Gray, Purple/violet, blue/green, Golden/bronze and green-gold. 100% Natural Colors: No enhancing nor bleaching done.