Polish bottom of dirty stone (4)
I believe the answer is:
ruby
'stone' is the definition.
(I know that ruby is a type of transparent gem)
'polish bottom of dirty' is the wordplay.
'polish' becomes 'rub' (polish meaning rub to make shiny).
'bottom of' says to take the final letters.
The last letter of 'dirty' is 'y'.
'rub'+'y'='RUBY'
(Other definitions for ruby that I've seen before include "Stony girl" , "Girl's name - colour" , "killed an assassin" , "Fortieth anniversary stone" , "Deep 6 across" .)