The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

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