Birthstone between sapphire and topaz (4)
I believe the answer is:
opal
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">birthstone between sapphire and topaz</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 "Opaque gem" , "Milky white gem" , "Translucent gemstone" , "Opaque mineral used a gemstone" , "Gem with delicate colours" .)