Quartz is having the most part of SiO2 (Silica) in it and formed in crystal form. A good quality Quartz can have silica upto 99.50%. Quartz is rigid like Diamond. Some formation of Silica (Quartz) is specific to Semi Precious Gems and its bulk industrial uses are in Glass amnd Ceramic manufacturing. Since the bonding is crystal bonding, melting temperature of Quartz is higher than the Feldspar.