Milky white stone (4)
I believe the answer is:
opal
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">milky white stone</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for opal that I've seen before include "Type of gem" , "Milky-white gem" , "Milky white gem" , "Translucent gem" , "Semi-precious stone of b;uish white colour" .)