Sandstone, Xuan Wuyan, limestone, granite which hard
But I think it is not absolute, because the sandstone can be divided into many kinds, such as quartz sandstone, high content of SiO2 is relatively hard, sometimes even harder than basalt, basalt and granite hardness similar, slightly hard granite (also mainly depends on the composition, basalt and granite also has many categories, such as Ayama Xuantakeiwa the granite and diorite..). But it is certain that limestone is one of the most soft.
Generally speaking, basalt and granite belong to igneous rock, the hardness is higher, and sandstone and limestone belong to sedimentary rock, which is relatively soft.
Basalt belongs to the extrusive rock in magmatic rocks, sandstone and limestone belong to sedimentary rock, granite belongs to magmatic rock, the most difficult in the above four kinds of rock
