Birthstone for most Libras (4)
I believe the answer is:
opal
'birthstone for most libras' 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 "One's stony" , "National gemstone of Australia" , "Lustrous stone" , "Milky stuff that's hard" , "it may be highly valued" .)