Compound in quartz (6)
I believe the answer is:
silica
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">compound in quartz</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)