Pieces of colourless transparent quartz (8)
I believe the answer is:
crystals
'pieces of colourless transparent quartz' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for crystals that I've seen before include "Clear quartz pieces" , "Clear transparent minerals, especially quartz" , "Rocks formed by the solidification of a substance" , "Try class to find pieces of high quality glass" , "Fragments of transparent quartz" .)