Polish bottom of heavy gemstone (4)
I believe the answer is:
ruby
'gemstone' is the definition.
(I know that ruby is a type of gemstone)
'polish bottom of heavy' is the wordplay.
'polish' becomes 'rub' (polish meaning rub to make shiny).
'bottom of' says to take the final letters.
The final letter of 'heavy' is 'y'.
'rub'+'y'='RUBY'
(Other definitions for ruby that I've seen before include "Girl's name - colour" , "Fortieth anniversary stone" , "Gemstone - shade of red" , "Stony girl" , "killed an assassin" .)