Disordered solids like glasses are also not minerals. The number of faces in a crystal is a diagnostic character. A crystal made up of like faces is termed simple form. Combined form is characterized by tow or more simple forms. They can t enclose space by themselves and called open forms.
Simple forms can enclose space by themselves and considered as closed forms.
An edge is formed by the intersection of any two adjacent faces. This the angel formed by the intersection of three or more faces. The angel between any two faces of a crystal. It is formed by the intersection of two normal lines taken along two adjacent faces. The interfacial angles are constant for all crystals if a given mineral with identical composition at the same temperature.