 Older mussels produce nacre more slowly, and second-harvest freshwater pearls (if not nucleated) are generally flat and often thin, with concavities and texture mostly absent from first-harvest pearls.Curiously, the Chinese use "keshi" to differentiate between two distinct products, while Japanese usage, requiring only the absence of a nucleus, is equally applicable to both.Because it is impossible to determine whether an individual keshi pearl grew serendipitously or as a result of mantle tissue insertion, they are all classified as cultivated/cultured pearls.