Compound in quartz (6)
I believe the answer is:
silica
'compound in quartz' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for silica that I've seen before include "Common base of rock" , "Compound in quartz" , "Hard colourless compound from rocks" , "Mineral found as quartz and opal" , "Quartz used in the manufacture of glass" .)