Quartz is a mineral composed of silica and oxygen (silicon dioxide - SiO2). Unlike the silica in glass, silica in quartz is crystalline. Quartz is the most common ingredient in most sand...and sand is the primary ingredient in glass.
Agate, amethyst, citrine, jasper, onyx, and opal are all varieties of quartz.
The melting temperature of quartz is over 2900F (1600C). It has a hardness of 7 on Mohs harness scale and a specific gravity of 2.65.