This is a tutorial for a granite like image.

First make a new image, I made mine 500x500px.

Now press D to reset the swatches to it's default colour.

Now apply a Clouds filter by going to Filters > Render > Clouds

Next go to Filter > Distort > Glass and use these settings


Now go to Image > Adjustments > Levels and increase the contrast so that it will look something like this.


After that go to Filter > Brush Strokes > Accented Edges and use these settings


Go to Filter > Brush Strokes > Angled Strokes now and use these settings


Next go to Filter > Stylize > Find Edges

And finally, go to Filter > Blur > Gaussian Blur with a setting between 1 and 2.

This is what I got with the above steps.