petrosilex
/ˌpɛtrə(ʊ)ˈsʌɪlɛks/Geology. Now rare
noun
Any of various hard, compact, igneous or metamorphic rocks rich in silica; especially (a) hornstone or chert; (b) felsite.
Origin
Late 18th century. From German Petrosilex from petro- + classical Latin silex flint, pebble.