Crossword Genius

Principal constituent of sandstone and other rocks (6)

Ross

I believe the answer is:

silica

(Other definitions for silica that I've seen before include "Main constituent of sand" , "Glassy substance in quartz and sand" , "Mineral found as quartz and opal" , "Main constituent of sandstone" , "Common base of rock" .)

I've seen this clue in The Independent.
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