Word: nacreous
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Dates: during 1920-1929
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Pearls fall into two main classes: true pearls and freshwater pearls. True pearls, or "orientals," are formed in oysters by the deposition of concentric layers of nacre, an iridescent substance, around a microbe or some other irritant. Freshwater pearls are formed in molluscs out of non-nacreous material, and are far less lustrous and valuable...