(as) hard as the nether millstone

• 铁石心肠

(as) hard as the nether millstone的用法和样例：

例句

1. They are merciless, with hearts as hard as nails.
   他们冷酷无情，铁石心肠。
2. The dictator was a man of great hardness of heart.
   那个独裁者是一个铁石心肠的人。