stone flax

例句：

1. 1. The ancient city is covered in a layer of smooth stones.
   这个古老的城市覆盖着一层光滑的石头。
2. 2. He found a beautiful stone carving on the wall.
   他在墙上发现了一块美丽的石头雕刻。
3. 3. Stones are often used as building materials.
   石头常被用作建筑材料。