Cameo stone (4)
(Other definitions for opal that I've seen before include "Form of hydrated silica, often used as a gemstone" , "Quartz-like gemstone" , "Gem-stone with a milky quality" , "Translucent gem" , "Milky-white gemstone with flashes of colour" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">cameo stone</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 onyx that I've seen before include "Smooth stone with bands of colour" , "precious stone" , "Hard stone with coloured layers" , "African antelope" , "material for making cameos" .)