On a smooth surface (gesso panel is ideal) spread a layer of Medium (no 2) . Into this with a medium size flat, put down the first layer of fissures, using black, raw umber and viridian. These are broad, few and big. Then, with a smaller flat (No 4) and adding raw sienna add the smaller fissures and fractures around the big ones.Vary the direction in different blocks and remember the perspective rules.
Now haze the surface, leaving some more definate. Into the spaces, add raw sienna , viridian , raw umber AND white mixtures with a stippling action, and hieghten (lighten) some smaller spaces. Take care to get a chiaroscuro to differentiate different blocks of marble. Haze again, and leave some definate fissures.
Now with the feather, add the hairline fractures using dark mixes, with black. Very judiciously , haze again.
Adding light and dark cut lines, and even cracks will add to the illusion. Use a mahl stick and a liner. Cissing (sprinkling with turps or metho) can work while its all still wet. This is a way to paint wet in wet, alla prima.. .. You should work out your perspective and draw it first. Light and shadow could be done first using a grizaille, or use glazes when its all dry.