sio2

Silex \Si"lex\, n. [L., a finit, a pebblestone.] (Min.)
   Silica, {SiO2} as found in nature, constituting quarz, and
   most sands and sandstones. See {Silica}, and {Silicic}.
   [1913 Webster]
    

Quartz \Quartz\, n. [G. quarz.] (Min.)
   A form of silica, or silicon dioxide ({SiO2}), occurring in
   hexagonal crystals, which are commonly colorless and
   transparent, but sometimes also yellow, brown, purple, green,
   and of other colors; also in cryptocrystalline massive forms
   varying in color and degree of transparency, being sometimes
   opaque.
   [1913 Webster]

   Note: The crystalline varieties include: amethyst, violet;
         citrine and false topaz, pale yellow; rock crystal,
         transparent and colorless or nearly so; rose quartz,
         rosecolored; smoky quartz, smoky brown. The chief
         crypto-crystalline varieties are: agate, a chalcedony
         in layers or clouded with different colors, including
         the onyx and sardonyx; carnelian and sard, red or
         flesh-colored chalcedony; chalcedony, nearly white, and
         waxy in luster; chrysoprase, an apple-green chalcedony;
         flint, hornstone, basanite, or touchstone, brown to
         black in color and compact in texture; heliotrope,
         green dotted with red; jasper, opaque, red yellow, or
         brown, colored by iron or ferruginous clay; prase,
         translucent and dull leek-green. Quartz is an essential
         constituent of granite, and abounds in rocks of all
         ages. It forms the rocks quartzite (quartz rock) and
         sandstone, and makes most of the sand of the seashore.
         [1913 Webster]