Ok the rock style has changed a bit on the detail-scale, but this is basically what I did.
I did a rock and all its details all on one layer.
START, MAKE A NEW LAYER.
1. By using a hard round brush with a colour above black and at 100% opacity, draw a thick straight line, preferably on an angle.
2. Erase lines by one or each end on an angle to give a sharp appearance. Make sure to straighten out the sides. Select the layer by ctrl+clicking the layer image and hide the lines by ctrl+h. This makes for no clean up!
3. Identify where the point is on the top end, and by using a smaller soft round brush at 50% opacity with a lighter colour, draw a line (using the point as a guide) all the way to the bottom of the shape. Doing this gives the shape depth and a 3D effect which is what needs to be achieved.
4. Using the same brush with the same opacity, but using a darker colour than your original shape's colour, start drawing lines on the left side of the guide line by the edge of the shape, following to the right until you hit your guide line. The furthest left line should be the darkest.
5. Use the same brush but select a colour lighter than your guide line, do the same process, but make sure to start at the guide line and not to overlap your lines too much.
6. Make your brush size a bit smaller and pick a very light colour, but not white. Overlap your guide line enough that the tip and a bit below are very light. Increase your brush size a bit and gently overlap the lighter lines around your guide line around the tip and a bit below it.
7. Make your brush size smaller and change the colour to white, or a super-duper light colour. Overlap the lines on the tip and a bit on the guide line until they basically become white or "shiny". Depending on the light source and where it is, you might even want to go left of the tip along the edge and whiten it as well, to give the effect of light bending over the tip. Press ctrl+d to deselect the layer.
YOU'RE DONE! This tutorial took me only ten minutes to make. You have the freedom to add more details as you wish!
Go right on ahead and use this tutorial if you wish, but please link me your work! I would love to see it!