Birthstone between sapphire and topaz (4)

I believe the answer is:

opal

'birthstone between sapphire and topaz' is the definition.
(I've seen this in another clue)

This is all the clue.

(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" .)

I've seen this clue in the LA Times, the USA Today and the L.A. Times Daily.

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