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" .)
'cameo stone' is the definition.
(I've seen this in another clue)
This is all the clue.
(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" .)