Crossword Genius

Polish unknown jewel (4)

Ross

I believe the answer is:

ruby

'jewel' is the definition.
(ruby is a kind of jewel)

'polish unknown' is the wordplay.
'polish' becomes 'rub' (polish meaning rub to make shiny**).
'unknown' becomes 'y' (algebra).
'rub'+'y'='RUBY'

(Other definitions for ruby that I've seen before include "Deep red variety of corundum" , "Jack perhaps" , "Girl's name - colour" , "Deep red - transparent gemstone" , "Fortieth anniversary stone" .)

I've seen this clue in The Mirror.
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