Crossword Genius

Impure form of quartz used in making pestles and mortars (5)

Ross

I believe the answer is:

agate

(Other definitions for agate that I've seen before include "marble" , "Quartz gem" , "Type of quartz" , "Striped stone" , "Ornamental form of quartz" .)

I've seen this clue in The Independent and The Mirror.
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