Stone for a Libra (4)

I believe the answer is:

opal

(Other definitions for opal that I've seen before include "Opaque mineral used a gemstone" , "Quartz-like gemstone" , "what may be set in 29 [DIAL]" , "Gem with delicate colours" , "Something milky-white" .)

I've seen this clue in the LA Times.

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