Here's why:

* Hardness: Limestone is a sedimentary rock, which means it's formed from the accumulation and cementation of sediments. Its hardness depends on the mineral composition and how well the sediments are cemented together.

* Mohs Hardness Scale: Limestone typically has a Mohs hardness of around 3 to 4. This means it can be scratched by a copper coin (hardness 3) but not by a fingernail (hardness 2.5). It's also softer than steel (hardness 5-6).

* Compared to other rocks: Limestone is considered a moderately hard rock, harder than soft rocks like chalk or gypsum, but softer than hard rocks like granite or quartz.

So, while limestone is not as hard as granite or diamond, it's still harder than many other common materials.