n. silicon dioxide, occurring as quartz etc. and as a principal constituent of sandstone and other rocks. silica gel hydrated silica in a hard granular form used as a desiccant. siliceous adj. (also silicious). silicic adj. silicify v.tr. & intr. (-ies, -ied). silicification n.
[ L silex -icis flint, after alumina etc. ]