To me, silex is name of my coffee maker. To Darwin it is a geolocical term, he used in describing some petrified tree trunks he found on a trip east of the Andes.
silex
n.
5L (gen. silicis), dissimulated < *scilec3 < *scelic3 < IE base *(s)kel3, to cut > SHELF, SHIELD6
1 silica, esp. in the form of flint or quartz