Polish unknown gemstone (4)
I believe the answer is:
ruby
'gemstone' is the definition.
(ruby is a kind of gemstone)
'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 "killed an assassin" , "Vivid red" , "Girl's name - colour" , "Deep red - transparent gemstone" , "Deep red variety of corundum" .)