Gem surface (5)
I believe the answer is:
facet
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">gem surface</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 facet that I've seen before include "Angle" , "Distinct feature" , "Smooth surface of a gem" , "Characteristic" , "Small surface (of diamond)" .)