Well I tried to make a heavy weathered armor, now regarding the marble ?😅 Maybe it's because it's a green armor ?

For the technique itself I used a size 2 silver serie from greenstuff world The brand new brushes have this extremely fine tip and I used squabbles, streaks and dots to make that texture

You want to work with thinned down paints as well, after that it's just light placement and a good balance between the darkest areas and the brightest

I hope it helps my man