Steelie alternative (5)
I believe the answer is:
agate
'steelie alternative' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for agate that I've seen before include "Quartz gem" , "Hard stone with concentric bands of colour" , "Stone with coloured bands" , "Quartz with bands of colour, used in jewellery" , "Old drama critic" .)