Crystalline, transparent (6)
I believe the answer is:
limpid
I believe this clue is a double definition.
'crystalline' is the first definition.
(I know that crystalline can be written as limpid)
'transparent' is the second definition.
(I know that transparent can be written as limpid)
(Other definitions for limpid that I've seen before include "Translucent" , "'Of liquid, clear and transparent (6)'" , "Crystal clear as water may be" , "Crystal clear like pure water" , "Crystal-clear" .)