The least expensive cultured pearls today rival the most expensive natural pearls ever found.
Natural freshwater pearls occur in mussels for the same reason saltwater pearls occur in oysters. Foreign material inside a mussel can't be expelled. To reduce irritation, the mollusk coats the intruder with the same secretion it uses for shell-building, nacre. To cultivate a pearl, farmers slit the mussel and insert small pieces of live tissue from another mussel.
The ancient Chinese practice this technique, but the first real cultured freshwater pearls originated in Japan. In the 1930's, Japanese farmers by Lake Biwa achieved natural colours unseen in saltwater pearls. However, water pollution today has virtually destroyed pearl production there.
Copying the Japanese to improve off-white and mottling, China uses a mild bleach, bright lights, and heat. Natural freshwater pearls are usually odd shapes. So for more roundness, the Chinese now reshape rejected pearls into spheres, and then nucleate mussels with them.
Freshwater pearls are popular for their colours: white, silvery-white, pink, red, copper, brown, lavender, purple, green, blue, and yellow. The most desirable are the pastel pinks, roses, lavenders, and purples. Colour comes from the mussel species and water quality. Generally pearls assume the colour of the shell in which they form.