Unit of weight for precious stones (5)
I believe the answer is:
carat
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unit of weight for precious stones</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for carat that I've seen before include "Measure of weight of diamonds" , "Weight of ice" , "Precious stone weight" , "Not a large weight" , "Measure of purity of gold" .)