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Nacre
This is also known as mother-of-pearl. Its chemical composition is:
° 90% calcium carbonate crystals (aragonite);
° 5% organic protein (conchiolin), which binds the aragonite crystals together;
° 2% water;
° Sodium, magnesium and strontium.

Natural Pearl
A real pearl naturally formed in an oyster without any human intervention.

Nucleus
This is the small, round piece of shell (bead) used by the grafter in the cultured pearl process. This bead carefully created from the shell of a Tennessee River-Mississippi River area mussel is introduced into an oyster to create a cultured pearl. (See Graft.)