Birthstone for most Libras (4)
I believe the answer is:
opal
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">birthstone for most libras</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 opal that I've seen before include "One's stony" , "National gemstone of Australia" , "Lustrous stone" , "Milky stuff that's hard" , "it may be highly valued" .)