Shimmery stone (4)
I believe the answer is:
opal
'shimmery stone' 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 "Opaque mineral used a gemstone" , "Milk-coloured gemstone" , "Milky-white gem" , "Opaque gem" , "One in a ring perhaps" .)