buhrstone 也作 burr.stone

n.（名詞）
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，砂質多孔石灰?guī)r：一種堅硬的，硅化石灰石，以前用來制作磨石。具有最初由化石化的貝殼而形成的多孔特征

來源：
Variant of burr
burr的變體
stone
stone