Agate (4)
I believe the answer is:
onyx
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">agate</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for onyx that I've seen before include "Agate with different layers of colour" , "Stone like marble with layered colours" , "Black and white striped gemstone" , "Stone-like marble with layered colours" , "Agate with alternating bands of colour" .)