Poles invested in low quality gem (9)
I believe the answer is:
moonstone
'gem' is the definition.
(moonstone is a kind of gem)
'poles invested in low quality' is the wordplay.
'poles' becomes 'ns' (North and South poles).
'invested in' is an insertion indicator.
'low' becomes 'moo' ('moo' can be a synonym of 'low').
'quality' becomes 'tone' ('tone' can be a synonym of 'quality').
'moo'+'tone'='mootone'
'ns' placed into 'mootone' is 'MOONSTONE'.
(Other definitions for moonstone that I've seen before include "Semi-precious gem described in monotones" , "something shiny" , "feldspar" .)