Crossword Genius

Hard mineral composed of silicon dioxide (6)

Ross

I believe the answer is:

quartz

(Other definitions for quartz that I've seen before include "Hard silica mineral" , "Type of watch and crystal" , "Mineral used in modern timepieces" , "Hard mineral in timepieces" , "Colourless mineral used to make glass" .)

I've seen this clue in The Guardian.
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