Feldspars (KAlSi3O8 - NaAlSi3O8 - CaAl2Si2O8) are a group of light pink, white, gray, brown rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust. Solid solutions between K-feldspar and albite are called alkali feldspar.They are relatively hard at 6 on Mohs' hardness scale.
Feldspar is used to make dinnerware and bathroom and building tiles. In ceramics and glass production, feldspar is used as a flux. A flux is a material that lowers the melting temperature of another material, in this case, glass.