Lustrous gem (4)

I think it could be either:

Which length description are you looking for?:

opal

'lustrous gem' is the definition.
(I've seen this in another clue)

This is the entire clue.

(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" .)

I've seen this clue in the King Feature Syndicate.

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