A variety of non-nucleated cultured pearls produced since World War II by means of the fresh-water mussel (Hyriopsis schlegeli) in Lake Biwa, Honshu, Japan.
Owing to the internal anatomy of the mussel, a solid foreign nucleus cannot readily be inserted, so that a small piece of mantle tissue (which later disappears) from another mussel is inserted in each of ten to twenty incisions in the mantle of the host mussel, which then, in about 1 to 2 years, produces the pearls.
They are brown or salmon coloured, and seldom spherical; they are bleached to a bright white. The process is repeated in some mussels, producing larger and darker baroque pearls.
Comparable non-nucleated cultured pearls have also been produced since 1958 in Australia by using a large variety of pearl oyster (Pinctada maxima).
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson