Here's why:

* Granite is an igneous rock, formed from cooled magma. It contains hard minerals like quartz and feldspar.

* Chalk is a sedimentary rock, formed from the accumulation of microscopic marine organisms. It's mostly made of calcium carbonate, which is softer.

You can test this yourself: try scratching a piece of granite with a fingernail or a piece of chalk. You'll find granite is much harder to scratch.