Chalcedony is the special name for quartz with microscopically small crystals. Unlike quartz, chalcedony does not look clear and glassy but translucent and waxy; like quartz it is hardness 7 on the Mohs scale. Unlike quartz it can take on every color imaginable. An even more general term, encompassing quartz, chalcedony and opal, is silica, the compound silicon dioxide (SiO2).
The major rock type that is defined by the presence of chalcedony is chert. Chalcedony also very commonly occurs as a mineral filling veins and openings—for examples see the pages for geodes and thunder egg.