Side of faceted gemstone (4)
I believe the answer is:
edge
'side' is the definition.
(edge is a kind of side)
'of faceted gemstone' is the wordplay.
'of' says the answer is hidden in the clue.
'EDGE' is hidden in the letters of 'faceted gemstone'.
(Other definitions for edge that I've seen before include "Golden ingot" , "Inch; border" , "'Border, boundary, rim (4)'" , "Margin, fringe" , "Border; intense quality" .)