What is a mineral used for making glass?

The two main constituents of glass are : Silicon Dioxide , Sodium Carbonate.

Alumina or borosilicate are added depending on the nature of the glass required.

Glass is a 'supercooled' liquid. , that is a liquid below its natural freezing point. Supercooled liquids do not have a crystalline structure, which pure solids have. However, glass if left for long periods of time (hundreds of years) may start to 'cloud', this is the crystals forming from the supercooled liquid.