Rock Textures from Scratch
This is simply a tutorial on one approach to creating rock textures from scratch.  The first step is to start out with a 256x256 (or whatever res you want) image and use Filter | Render | Clouds.  This filter is handy because the resulting image can be tiled without any seams:
Cloud Filter

Now, for a more detail, use the Difference Clouds filter (Filter | Render | Difference Clouds).  Repeat this many times.  Just keep hitting Ctrl-F.
Difference Clouds Filter

In the channels tab, right click on one of the channels and select "Duplicate Channel."  Select the RGB channels, and fill them with a grey (select a grey background color, then hit ctrl-a, then del).
Duplicate channel

The cloud effects will be used as a bumpmap for the rock.  Some cool effects can be achieved using the lighting effects: Filters | Render | Lighting Effects...
Play around with it until you get what you want.  It's always a good idea to save your lighting style once you've found one you like, because they're really hard to replicate. :)
Lit texture...

There are some really strong highlights and shadows in this one, which we don't want (it makes the tiling really noticeable).  This can be reduced by copying and pasting the image into a new layer, making it about 50% transparent, and offsetting it, or by repeating the procedure in a new image and pasting that on top.

It needs some color now.  Make a new channel and fill it with noise (Filter | Noise | Add Noise).  We'll use this channel for speckling a variety of color on the rock.   Blur it a little to soften the noise.
Blurred noise...

Create a new layer in the RGB channels and fill it with what you want to use as the base color of the rock.  I used brown, but you might want to use more of a grey, depending on what kind of a rock texture you're trying to create.  Once you've done that, control-click on the channel you've created -- the one with the noise.  This will select based on the values in that channel.

Set another color as your background color.  I used a different shade of brown.  You might try experimenting with some pale greens for a mossy look.

Now set that layer to hard light (or possibly soft light or multiply, depending on what looks best with what you've done).
Hard Light...

At this point, it's just a matter of tinkering to adjust it the way you want to, or you can just leave it at that.  Before tinkering, it's best to copy all of the layers merged into a new layer (Ctrl-A, Ctrl-Shift-C, Ctrl-V).  Then you can tinker with the brightness and whatnot.

If you want to make more defined rocks or more of a stone-wallish look, throw in another layer set to multiply and start defining some cracks/shadows with the airbrush tool using a dark or black color.  A drawing tablet can be useful in this stage:
Layer 5

Highlight them as well with a color dodge layer (using a light color):
Layer 6
Note: Never work all the way to the edge of the image.  Work in the middle, then offset all of the layers you're working with.

The first result of this technique...
Final Texture

It didn't come out as well as I had hoped, but that's what I get for trying to put together tutorial images as I experiment.  Here's one highlighted a little differently:
Better Result...

Feel free to contact me about anything in this tutorial that didn't make sense or could use improvement.  Heck, you can even just tell me about your pet wombat for all I care.  I hardly get any real (non-spam) email anymore.

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Created Oct. 31, 2002.
Content on this page is Copyright © 2002 Nathan "jitspoe" Wulf, all rights deserved.