Limpid, crystalline (8)
I believe the answer is:
pellucid
I believe this clue is a double definition.
'limpid' is the first definition.
(I know that limpid can be written as pellucid)
'crystalline' is the second definition.
(I know that crystalline can be written as pellucid)
(Other definitions for pellucid that I've seen before include "Absolutely crystal clear" , "Translucently clear" , "Translucent" , "Very clear, totally transparent" , "Crystal-clear" .)