The marble-like substance of some stony cross (4)
I believe the answer is:
onyx
'substance of some stony cross' is the definition.
'onyx' can be an answer for 'substance' (onyx is a kind of substance). I am not sure about the 'of some stony cross' bit.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for onyx that I've seen before include "Smooth stone with bands of colour" , "material for making cameos" , "Marble-like gemstone" , "Stone like marble with layered colours" , "Agate with alternating bands of colour" .)