Marble type (5)
I believe the answer is:
agate
'marble type' 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 "Semi-precious quartz" , "On the way" , "Quartz with bands of colour, used in jewellery" , "Type of quartz" , "Onyx" .)