Opaque impure form of quartz often red, yellow, green or brown in colour (6)

I believe the answer is:

jasper

(Other definitions for jasper that I've seen before include "hard matter" , "Gemstone" , "Opaque reddish-brown semi-precious stone" , "Reddish-brown stone used in jewellery" , "Opaque quartz" .)

I've seen this clue in The Mirror.

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