In the rocks cycle, silica forms a tetrahedron of silicon and oxygen. It has a melting temperature just at the temperature of supercritical steam from volcanic vents, so as the lava or magma cools, silica will be one of the last things to crystallize out (along with heavy metals like gold and platinum).In a very wet volcanic melt, the silica glass will be an wruptive material called obsidian. This is a true glass because its not a crystal and is plastic at all temps
With weathering the silica (natural glss or crystal) will be eroded as a sand particle .
Glass can also be formed as a biological skeletal framework that occurs in several species of plants like grasses and certain trees and . When these are compressed in sediment bands, they can form deposits of chert, or silica lenses. These , in turn, will further break down into sands.
Commercial glass is a mixture that Francis presented . Other stuff is added to the mix to impart other properties, colors , or chemistry.