Silex

Silex is any of various forms of ground stone. In modern contexts the word refers to a finely ground, nearly pure form of silica or silicate.
In the late 16th century, it meant powdered or ground up "flints" (i.e. stones, generally meaning the class of "hard rocks") 
It was later used in 1787 when describing experiments in a published paper by Antoine Lavoisier where such earths are mentioned as the source of his isolation of the element silicon. "Silex" is now most commonly used to describe finely ground silicates used as pigments in paint.