Gem displays bottom quality (9)

I believe the answer is:

moonstone

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'gem' is the definition.
(moonstone is a kind of gem)

'displays bottom quality' is the wordplay.
'displays' becomes 'moons' (mooning is a kind of displaying).
'bottom quality' becomes 'tone' (synonyms. I am not sure about the 'bottom' bit.).
'moons'+'tone'='MOONSTONE'

Can you help me to learn more?

(Other definitions for moonstone that I've seen before include "feldspar" , "something shiny" , "Semi-precious gem described in monotones" .)

I've seen this clue in The Independent.

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