Here's why:
* Igneous rocks form from cooled and solidified magma or lava. They often have a crystalline structure, with tightly interlocked mineral grains, which makes them very strong and resistant to scratching. Examples include granite, basalt, and obsidian.
* Sedimentary rocks form from the accumulation and cementation of sediments. They are often porous and have weaker bonds between their mineral grains, making them less hard than igneous rocks. Examples include sandstone, limestone, and shale.
* Metamorphic rocks form when existing rocks are transformed by heat and pressure. Their hardness can vary widely, depending on the original rock and the degree of metamorphism. Some metamorphic rocks like marble (from limestone) are harder than the original sedimentary rock, but others like slate (from shale) are softer.
While there are exceptions, igneous rocks generally tend to be the hardest due to their crystalline structure and the processes involved in their formation.
