Regularly burnished gemstones, essentially yet to be mounted (5)

I believe the answer is:

unset

'yet to be mounted' is the definition.
(not yet set or mounted)

'regularly burnished gemstones essentially' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'essentially' indicates the central letters.
The middle of 'gemstones' is 't'.
The alternating letters of 'burnished' are 'unse'.
'unse'+'t'='UNSET'

I've seen this clue in The Guardian.

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