The key requirements for a cutting tool are that it should be sharp and durable. Minerals are not very suitable for toolmaking because of their crystalline structure. When a crystal is broken, it cleaves primarily along certain faces. Such cleavage does not generally produce a sharp edge. So we don't want to use a mineral for toolmaking.
Heterogeneous rocks consist of interlocking crystals of different minerals.
When broken, such rocks will split between the different crystals, and again a dull edge is produced. A homogeneous rock generally lacks an overt crystalline structure. It can be broken in any direction, not just along crystal faces. There are many homogeneous rocks and they vary considerably in their mechanical properties. For our purposes, we need a hard rock, that is, one that is not easily scratched.