Form of quartz (6)
I believe the answer is:
jasper
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">form of quartz</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for jasper that I've seen before include "Semi-precious form of quartz" , "hard matter" , "A man" , "Reddish-brown stone used in jewellery" , "stone used by jewellers" .)