Iridescent gemstone (4)
I believe the answer is:
opal
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">iridescent gemstone</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for opal that I've seen before include "Milky stuff that's hard" , "Lustrous stone" , "Precious stone" , "Type of gemstone" , "Iridescent gemstone" .)