A mineral is a naturally occurring solid that is made up of very specific types of atoms (elements) arranged in a very specific way.
The atoms inside a mineral are arranged in a very specific pattern. Materials made of very specific kinds of atoms arranged in a very specific pattern are said to be crystalline. The three-dimensional space in which atoms are arranged in a continous and orderly way is a single crystal. Crystals grow larger by adding more atoms on their outer edges.
Because of their very specific composition and internal order, specific minerals behave in very specific ways.