nacred

• Mr. Gray justly observes they are merely the internal nacred coat of the shell, which has been forced, by some extraneous cause, to assume a spherical form. —  The Mirror of Literature, Amusement, and Instruction Volume 20, No. 570, October 13, 1832
• That, in many instances they are formed by the oyster, to protect itself against aggression, is evident; for, with a plug of this nacred and solid material it shuts out worms and other intruders which have perforated the softer shell, and are intent on making prey of the hapless inmate: and it was apparently the knowledge of this fact that suggested to Linnaeus his method of producing pearls at pleasure, by puncturing the shell with a pointed wire. —  The Mirror of Literature, Amusement, and Instruction Volume 20, No. 570, October 13, 1832

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