(materials) A dense, more or less round, white or light-colored concretion having various degrees of luster formed within or beneath the mantle of various mollusks by deposition of thin concentric layers of nacre about a foreign particle.
(pathology) Rounded masses of concentrically arranged squamous epithelial cells, seen in some carcinomas. Mucous casts of the bronchi or bronchioles found in the sputum of asthmatic persons.

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