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.

1. Rounded masses of concentrically arranged squamous epithelial cells, seen in some carcinomas.

2. Mucous casts of the bronchi or bronchioles found in the sputum of asthmatic persons.