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 Various

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 Various