A tough, silicified limestone formerly used to make millstones. It is typified by the presence of multiple cavities that originally housed fossilized shells.

• 磨石，砂质多孔石灰岩一种坚硬的，硅化石灰石，以前用来制作磨石。具有最初由化石化的贝壳而形成的多孔特征