This tutorial is 3 steps and has a difficulty of Intermediate. This tutorial will show you from start to finish how to make a realistic looking stone texture that you may see on castles, boulders, mountains, etc. The result is pictured below.
Step 0

     1. First create a new document, I'm going to use a 300 x 300 pixel image ok. All colors in this tutorial are in hexadecimal (hex) value.

     2. Now, chose a good light gray color for the foreground color, something like D3D3D3 will do nicely. Also pick a nice dark gray color for the background color, soemthing like 3D3D3D should work well.

     3. Now you need to go to Filter > Render > Couds. You should now have something like the below image.
Step 3

     4. Create a new channel for your image by clicking the channels tab on the layers window. If your layers window is not up open it by pressing Window > Layers. Now at the bottom of the channels tab of the layers windows click the "Create new channel" button to the left of little trash can. The channels tab should look like this after you make the channel (except the new channel should be just black).
Step 4

     5. Click the new channel you just created, which is filled with blackness, then go to Filter > Render > Difference Clouds, yes it is gray scale even if you were working with a colored image don't worry. Now press Ctrl + F or just keep going to filter difference clouds, untill you get something like the below image for your channel.
Step Five

     6. And now for the final step. Click Filter > Render > Lighting Effects. Click the Texture Channel drop down box near the bottom left and select that channel you made earlier. Now just play around with the other settings, or use the ones I did in the below image.
Step Six

Finished