I'd google marble surfaces and have a look at the different kinds of marble and the way the lines work... if there is a general pattern or a shape the lines have more often, then get a sheet of paper or spare plastic and start trying. From the pictures that Eye of Error posted, I'd suggest just about what he said. If you shade the surface before painting the lines on make sure to make another glaze / shading layer afterwards so that the lines look like inside the stone, not on top of it.
my 2ct