Key material (5)
I believe the answer is:
ebony
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">key material</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 ebony that I've seen before include "Hard wood used for black piano keys and in cabinetwork" , "Tree having black wood" , "Very dark brown" , "Very dark black" , "Hard block tropical wood" .)