Lustrous gem (5)
(Other definitions for pearl that I've seen before include "greatly valued" , "Gem from oyster" , "prized example" , "Lustrous globule prized as a gem" , "Off-white colour" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">lustrous gem</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 gemstone" , "Gem-stone with a milky quality" , "Semiprecious stone of white and bluish colour" , "Iridescent quartz-like stone" , "Milk-coloured gemstone" .)