Stroke unknown stone (4)
I believe the answer is:
ruby
'stone' is the definition.
(I know that ruby is a type of transparent gem)
'stroke unknown' is the wordplay.
'stroke' becomes 'rub' (I've seen this in another clue).
'unknown' becomes 'y' (algebra).
'rub'+'y'='RUBY'
(Other definitions for ruby that I've seen before include "Red precious stone" , "A red jewel" , "Precious red gem" , "Vivid red" , "Stony girl" .)