Gem and Ornamental

154 GEMS AND OKNAMENTAL ~~ OF ORGANIC ORIGINFqure 9.11 Composite nucleated cultured blister pearl (hbt'), showingthe mother-of-pearl back (mqnihl].and the success rate is not high. For example, Akoyas will produceabout 200 saleable pearls and 50 top-quality ones hm 1000 graftedoysters.Some species of oyster, for example the South Sea pearl oyster, canbe nucleated a second time, immediately after the h pearl has henremoved, by inserrjng a new bead into the pearl sac already there.Akoyas can only be nucleated once.Marine pearl o m that are farmed are all nucleated with a beadand a piece of mantle tissue. Freshwater mu~sels that are farmed areimplanted with mantle tissue grafts only. In some areas the oysters andmussels to be used are bred and cultivated until they reach maturityand can be implanted. In other areas oysters are caught in the wild tobe cultivated.As already stated, a mollusc can also cwer an object that is attachedto the inside of its ahell. Tlis can be a natural occurrence, but is alsocopied to produce cultured blister pearls, A hemisphere of moth=-of-pearl or plastic is amched to the shell undernea~ the mantle, and themollusc is left to coat the object with nacre. Usually the blistex pearlwill be cut away from the bell and the shell back md nucleus removed,leaving a hollow hemi~phere of nacre. This is cleaned, and filled withresin or wax. The hemisphere is then backed with a new piece ofpolished mother-of-pearl. The mult is a composite cultured bli~terped. The term 'mabt' is now generic md used to demibe thesepearls, although, strictly speaking, it should only refer to blister pearlshm a single species of oyster - the black winged oyster (Figs 9.3 and9.11).