Pearl farms in China today utilize hatchery born oysters. These oysters are of two varieties; the Pinctada fucata, and the Pinctada imbricata. Young, one-year-old oysters are used to cultivate the smaller pearls of 3-5mm. These oysters are typically only 6 centimeters across. Slightly larger oysters, 2-3 years of age and 7-8 centimeters across, are used to cultivate pearls 5mm and larger.
Nucleation is done throughout the year, but the most popular times are from March to July. Before the nucleation process the oysters must be ovulated and thoroughly cleaned. Each oyster is typically nucleated with two nuclei. The nucleus is placed inside the oyster’s gonad with a small piece of mantle tissue – epithelial cell side out. The size of the nucleus is determined by the size of the oyster, with a small oyster of 30 grams accepting on small 5-6mm nuclei, and a larger oyster of 45 grams accepting a 7mm nucleus.
Immediately after nucleation the oysters are returned to a still water environment where they heal after this major surgery. This healing process lasts at least one month, and the mortality rate is highest at this time. Typically 30% of the oysters do not survive this first month.
After a three months of healing the oysters are then x-rayed to determine whether the nucleus has been jettisoned. If the oyster does not contain a nucleus it may be re-nucleated, or discarded if deemed unfit. The oysters that appear to be healthy and still holding onto their gems and then placed back into the sea, where they are suspended up to 2 meters below the surface. Many farms use permanent culture rafts, while many farms (particularly popular in Guangdong Province) use the stake-line method.
The temperature of the water is then constantly monitored for the next 1.5 years. The optimum temperature in the summer months is no higher than 28 degrees Celsius, and no lower than 12 degrees Celsius in the winter. Many of the larger farms will raise or lower their crop during these times to find the optimum depth for nacre secretion. They will also, one month before harvest, lower the oysters to a depth of up to 4 meters. At this depth the cooler water slows the metabolism of the oyster and in effect slows the rate of nacre secretion. This last month of layered nacre is important to produce a top-coat with the highest possible shine and fine texture.
Farmers typically specialize in a particular size of pearl. With each size the rewards differ, as do the risks and costs. A farmer will pay approximately 4 cents for a one-year-old oyster, but will pay up to 25 cents for a two-year-old. The mortality rate is higher for larger oysters as well. So the risk and cost of producing smaller pearls is much lower, but the prices fetched at harvest typically follow the same rules.