Milky gemstone (4)
I believe the answer is:
opal
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">milky gemstone</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 opal that I've seen before include "Delicately coloured gem" , "Iridescent quartz-like stone" , "Semi-transparent gem" , "Milky-white gemstone with flashes of colour" , "Type of gem" .)