Entry 1 sense Webster, 1913 Silex /sĭl'-əks/ · Si·lex · IPA /ˈsaɪˌlɛks/ 01 n. Silica, SiO2 as found in nature, constituting quarz, and most sands and sandstones. See Silica, and Silicic. 1. Silica, SiO2 as found in nature, constituting quarz, and most sands and sandstones. See Silica, and Silicic.(Min.) See: Silica, Silicic