as hard as the nether millstone

例句：

1. 1.The road was so hard that it was almost impossible to drive on.
   这条路太硬了，几乎不可能开车。
2. 2. He is as hard as a rock when it comes to his work.
   在工作上他非常固执，就像岩石一样。
3. 3. The rocks were so hard that they could not be moved by hand.
   这些石头太硬了，不能用手搬动。