This rock is made up of "minerals. Minerals are made up of atoms, which are very fine particles that are arranged in a regular pattern and stuck together. The force with which these atoms stick together is what makes minerals hard.
The rock formed by the cooling of magma ejected from a volcano is called "igneous rock." When igneous rock is formed, the mineral particles are firmly interlocked and stick together. Since the minerals are originally hard, it is only natural that they are hard.
Some rocks are sedimentary rocks, which are formed when dirt, sand, and pebbles accumulated at the bottom of the ocean in ancient times. These rocks were crushed and clumped together by the weight of dirt and sand that accumulated on top of them over millions, tens, or hundreds of millions of years. Because they are crushed by the tremendous weight over a long period of time, the crushes of dirt and sand stick together tightly and become very hard.