Step 1:  Create a blank 200 by 200px document and use the horizontal type tool to create a single letter. In my case I've used the 'Tarzan' font at size 222. Don't worry if you don't have this particular typeface - just select something with a chiselled look (it'll look more realistic when overlaid with a stone texture).   You don't have to use a letter, of course - it'll work on any stone effect interface.

Step 2: Create a new transparent layer on top of the other layers and, using the brush tool (with airbrush if needed), and eraser tool, draw some cracks.  Take your time with this stage - the more time you take working on the fine detail, the better your end result.  Try to avoid using a brush that is too big or you'll notice that its an artificial effect right away!   When you are finished, change the layer blending mode to Hard Light and drop the opacity to 75%.

Step 3: With the black cracks layer selected, pick Layer > Duplicate from the menu to create an exact copy.  Now select the lower layer and choose Image > Adjustments > Inverse.  You should end up with white and black cracks on two separate layers, although you shouldn't be able to see the white because it'll be overlapped by the black.  Using the cursor keys, offset the white cracks 1px down and 1px to the right.  As you have probably guessed by now, the perception of depth is just an illusion...

Step 4: Now, using layer effects or texture mapping, give your text / interface a more pleasing look.  Et voila - your finished design, slightly worse for wear. Of course, you don't have to limit yourself to cracks - with a little more work you can create crevices, overhangs, and other natural phenomena. Studying photos of stressed rock can also help you make the effects a little more realistic.  Just have a play around - its often surprising how such a simple effect can be so powerful.