To apply the distinctive vein lines that are so much a part of the characteristics of most marble, you can use the edge of a feather or a fine liner paint brush. Dip your feather or brush lightly into your high contrast glaze (mine is almost black)and then wipe off most of the glaze on a piece of paper towel. Draw the edge of your feather gently across your surface, pulling fine lines diagonally across your marble. The feather 'bounces' as you use it, leaving realisitic gaps in the fine veins. Waggle the feather as you pull it through the paint to make the vein lines wider and narrower in places. If you use a brush, make sure your veins are tiny, and slightly irregular, with a few gaps in the paint.