Black gem (4)
I believe the answer is:
onyx
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">black gem</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for onyx that I've seen before include "Semi-precious stone used in making cameos" , "Stones" , "Kind of agate" , "Cameo material" , "Form of agate" .)