* Limestone: This is the softest of the four. It's a sedimentary rock made from calcium carbonate.

* Marble: This is a metamorphic rock formed from limestone. It's harder than limestone due to the transformation process but still relatively soft.

* Granite: This is an igneous rock, formed from cooled magma. It's harder than limestone and marble, thanks to its mineral composition (mostly quartz and feldspar).

* Diamond: This is a pure form of carbon, and it's the hardest naturally occurring material on Earth. It's significantly harder than any of the rocks listed above.

In summary: Diamond is the hardest, followed by granite, marble, and then limestone.