You can create a marble texture easily with a couple of filters: Clouds and Difference Clouds. Choose black for the Foreground and green (or any marble-like color, other than white) for the Background color (at the bottom of the Toolbox). Go to the Filter menu and choose Render>Clouds, then choose Filter>Render>Difference Clouds.
Press Command-F (PC: Ctrl-F) a few times to reapply the Difference Clouds filter until you get the desired look. from Bert Monroy