When it comes to hardness, metamorphic rocks often take the lead. This is primarily due to the immense pressure and temperature they undergo during their formation. For instance, rocks like granite, which is an igneous rock, are quite hard, but when subjected to metamorphic processes, they can transform into something even tougher, like gneiss.
On the other hand, sedimentary rocks, such as sandstone or limestone, are typically softer because they are formed from the accumulation of sediments and organic materials. These processes do not create the same density and crystalline structure found in igneous and metamorphic rocks.
In conclusion, while both igneous and metamorphic rocks can be hard, metamorphic rocks generally exhibit the greatest hardness due to their complex formation process involving high pressure and heat.