* Shale: Shale is a fine-grained sedimentary rock composed of clay minerals and silt. Its hardness can vary depending on the composition and degree of compaction, but it is generally harder than sandstone.

* Sandstone: Sandstone is a sedimentary rock made up of sand-sized particles. It is typically softer than shale due to the larger size and less compact nature of its grains.

* Limestone: Limestone is a sedimentary rock primarily composed of calcium carbonate. It is generally softer than both shale and sandstone, especially when it is porous or contains significant amounts of clay.

Hardness Comparison:

* Shale: Mohs hardness scale: 2-4

* Sandstone: Mohs hardness scale: 1-3

* Limestone: Mohs hardness scale: 3

Other factors affecting hardness:

* Compaction: The degree of compaction can significantly affect the hardness of sedimentary rocks. More compact rocks are generally harder.

* Cementation: The presence of cementing minerals, such as silica or calcite, can increase the hardness of sedimentary rocks.

* Mineral composition: The specific mineral composition can also affect hardness. For example, quartz-rich sandstone is generally harder than feldspar-rich sandstone.

Conclusion:

While hardness can vary within each rock type, shale is typically the hardest among the three. Sandstone is generally softer than shale, and limestone is the softest.