Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Source : WordNet®

silex
     n 1: a pure form of finely ground silica
     2: a vacuum coffee maker