The distinction between rocks and minerals follows from our discussion of pure substances. The term rock is used to describe a naturally occurring mixture of minerals. The mixture may be either homogeneous or heterogeneous. Crystals of pure substances may be grown, either by slowly cooling molten material or by slowly evaporating a solution. There are interesting exceptions to this rule, but it holds for most pure substances.
Pure substances have a melting point .
Impure substances melt over a range of temperatures. Again, there are interesting exceptions but they are beyond the scope of our present discussion. When discussing rocks, then, we will refer to the minerals present in the rock. When discussing minerals, we will refer to the chemical composition, crystal structure, and melting point. Our immediate concern is with materials suitable for making tools.