Real Dictionary

silica
uncountable noun: Silica is silicon dioxide, a compound of silicon which is found in sand, quartz, and flint, and which is used to make glass.

silica in American English
a glassy, very hard mineral, silicon dioxide, SiO2, found in a variety of forms, as in quartz, opal, chalcedony, sand, or chert
noun: the dioxide form of silicon, SiO2, occurring esp. as quartz sand, flint, and agate: used usually in the form of its prepared white powder chiefly in the manufacture of glass, water glass, ceramics, and abrasives

silica in British English
noun: the dioxide of silicon, occurring naturally as quartz, cristobalite, and tridymite. It is a refractory insoluble material used in the manufacture of glass, ceramics, and abrasives