Diamond surfaces (6)
I believe the answer is:
facets
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">diamond surfaces</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 facets that I've seen before include "Surfaces or aspects" , "Surfaces of a cut gem" , "Sides or surfaces" , "Faces of cut gemstones" , "Sides or aspects, diamonds or buildings" .)