I seem to remember seeing a kit for doing this sort of thing at our local craft supply store.
It was called marbling or marblizing or something like that.
I had some panels done a long time ago by a guy who painted wood to look like marble.
He started with a dark blue basecoat then painted veins into it using oil based paint, applied with the tip of a feather, next he flicked white spirit all over it and smudged it all about using a large very soft badger hair brush.
the whole thing was then coated in varnish after a couple of days drying time.
All the time he was painting he had a pint of beer in his left hand, said it was to balance out the weight of the feather ! all I know is that after doing 8- 8' x 4' panels the effect became much more random