Second harvest non-nucleated China freshwater pearls (aka Chinese fw keshi)
These pearls differ from those that grow during a mussel's youth, in that their shapes are less full - instead they have concavities, texture and altogether different character. The more mature mussels grow much more slowly than at the peak growth stage mostly utilized for pearl cultivation. This means that, though a pearl as large in area as the pearl sac will eventually grow, it will tend to remain thin for a long time. Unusual styles of processing, eg eccentric tangentially-drilled "petals" are needed to utilize pearls that are beautiful and lightweight, but too thin to string or set in traditional ways.
Harvesting this type of pearls requires only that care is taken to keep the mussel alive during the first harvest, and for some years thereafter until the crop is deemed ready. However, by adding the operation of inserting a bead or flat shell piece into appropriately shaped pearl sacs, immediately following the first harvest, flame-type bead nucleated pearls and coin pearls can be produced simultaneously with these non-nucleated second harvest pearls. Now part of the same production stream, it is no longer unusual to find pearls with and without nuclei mixed in strands. Since mussels may live many years, the harvesting (with and without insertions of nuclei) can be repeated, but the yield keeps diminishing with the natural slowing of nacre production as mussels reach adult size. Chinese producers get 3 harvests of mantle-cultivated pearls from most of their mussels. Since it is presently not possible to distinguish between pearls of second and subsequent harvests, we include all of these in our definition of second harvest pearls.
A new type of pearl, called non-nucleated or keshi by most sellers, appeared around the end of 2008. Generally thicker than the non-nucleated second harvest pearls but thinner and more curvy than baroque coin pearls made with flat shell nuclei, they appear to be distinct from both of those types. For more details, please see: lightweight coins. Second harvests from pearl sacs that have produced coin pearls tend to have more regular shapes and not to require special drilling. With all these improvements, there are fewer old-style thin keshi on the market.
For background and details of the use of terms such as keshi and Biwa for freshwater pearls, please read freshwater pearl naming.