... that common glass is made from about 70%
amorphous silicon dioxide, which is the same chemical compound
found in quartz (and its polycrystalline form: sand).
The term glass refers to compounds based on
silicon and oxygen, and ordinary soda-lime glass, used in
windows and drinking containers, is created by the addition
of soda and lime (calcium oxide) to silicon dioxide.
Without these additives, and a slow
cooling process, silicon dioxide will form quartz crystals,
not glass!