Opal, essentially (6)
I believe the answer is:
silica
'opal essentially' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for silica that I've seen before include "Quartz used in the manufacture of glass" , "Common base of rock" , "Compound in quartz" , "Main constituent of sandstone" , "Hard colourless compound from rocks" .)