It must be naturally occurring. It must be a solid. It must have a definite chemical structure. A Rock is simply a variety of minerals put together. Minerals are the building blocks that make up rocks.
Minerals have several different ways of being identified.
There is about 4000 different kinds of minerals known to man and of these known only 3 dozen of them make up the majority of the Earths crust. Well I will tell you it is easier than it sounds. When they find a mineral they put it through a series of basic physical tests, described below, to figure out the mineral's identity. Luster is the mineral's ability to reflect light and either have a metallic finish such as pyrite or a nonmetallic look, such as the vitreous or glassy look of quartz. A mineral's streak is the color it appears in a powdered form.