152	Gem Trader
	where it has been fished by Arabs since early times in primitive fashion. It is only quite recently that the Au­stralian pearling grounds were discovered, perhaps fifty or sixty years ago, but the pearls found there, though often very fine, are quite different from the Oriental pearls, and the oysters out of which they come are of an­other kind. The Japanese pearl oyster is different again, and not a producer of good pearls or good shell. But the Japanese pearl oyster has the distinction of being the step­mother of the cultured pearl. Nowadays, almost the first question a pearl merchant is asked is: "What is a cultured pearl?" and next: "Can you tell the difference?" A cultured pearl is made by introducing, in a special way, a foreign body into a living oyster's shell. If the foreign body is very minute, it stands the same chance of being covered evenly and well with nacre so as to produce a fine pearl just as any other for­eign body, accidentally introduced. That is, perhaps a ten thousand to one chance. In such a case it would be as a "real pearl", indistinguishable from any "natural" pearl, although tending to be second class, as most Japanese pearls are. In any case, its sheen and lustre would show where it had come from. But cultured pearls started on very tiny cores are not a commercial proposition, and it is the rule to insert a core of some size and spherical in shape so that a largish round pearl can be produced in a reasonable time, for it takes years for the oyster oblig­ingly to deposit the thin layers of nacre on the pearl. Thus the expert can always tell the cultured pearl from others because it usually consists of a small bead coated