How do you get haze out of the cracks in black slate?

I am tiling a bathroom and have gotten the haze out of most of the black slate, except for the cracks. I've used a brush to scrub them, but I can't seem to get the haze out of the cracks. It's real slate (stone). I sealed it before installing the tiles, but I guess I didn't seal the cracks well enough. Any ideas?