Gazes vacantly at shade of gem (9)
I believe the answer is:
moonstone
'gem' is the definition.
(moonstone is a kind of gem)
'gazes vacantly at shade' is the wordplay.
'gazes vacantly' becomes 'moons' (I can't justify this - if you can you should give a lot more credence to this answer).
'at' is a charade indicator (letters next to each other).
'shade' becomes 'tone' (synonyms).
'moons'+'tone'='MOONSTONE'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for moonstone that I've seen before include "Semi-precious gem described in monotones" , "feldspar" , "something shiny" .)