October gem (4)
I believe the answer is:
opal
'october 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 "Milk-coloured gemstone" , "Semi-precious stone of b;uish white colour" , "Type of gem" , "Gem-stone with a milky quality" , "Opaque gem" .)