Gem and Ornamental

'Shell'). The pearls are iridescent, with the same blues, greens, purplesand pinks that me displayed by the shells. The pearls are natural, andare usually hollow and of unusual shape - often cusp shaped - andvery seldom round (Fig. 9.8). Nowadays cultured composite blisterpearls are being produced in abalones.Non-nacreous pearlsThere are several other molluscs that can, on occasion, also producepearls. The most famous are the conch and the baler, which bothproduce pearls of peat value, although they are non-nacreous. Others,such as he horse conch and the noble pen shell, are less well known.Some molluscs produce concmions that are called pearls, but whichtend to be curiosities rather than beautiful items. The most famous isthe clam.The conch, &mn&cp gigas, is a gastropod. The lining of a youngshell is a peachy- oolour, becoming strong pink as the molluscreaches maturity. When viewed under mation the surfacedisplays a Welike pattem, caused by the striated appearance of thearagonite crystals that make up the inner layer of the shell. This patternis repeated on the pearls (Fii. 9.9).Conch pearls are very rare, slightly oval in shape, and are seldommore than 8 milhems long. They have the same orange or pinkFigure 9.9Conch pearl, showing "8arnea pattern (mmed).