Quartz is a mineral composed entirely of silicon dioxide. Both the "rock crystal" form and the "milky" form are particularly pure.
Other kinds, such as rose quartz, smoky quartz, and amethyst, contain iron impurities which make them unsuitable for some applications such as glassmaking
Quartzite and Chert
Quartzite and chert, like quartz, can come in clear and white varieties, which are relatively pure silicon dioxide, and colored varieties, usually containing iron or other impurities. For some applications they can be substituted for quartz.