For those interested, I've written a step-by-step with photos, below.
I started out with a base of watercolor paint, using the wet-into-wet technique. I got the paper very wet and dropped one color at random, cleaned my brush, dropped another color at random and repeated this with different colors until the page was almost completely colored.
I didn't worry about which colors to use, where to drop them in, or what size or shape the drops were making. (Note: when I say dropped, I'm referring to the technique that's called 'charging' in watercolor. Depending on how watery your paint is, you may actually 'drop' paint onto the paper, or more likely, you may touch the tip of the brush to the paper and then quickly lift it, leaving just a touch of color).
After the paper dried thoroughly (you can ruin your Micron pen if you draw on wet paper), I began looking for rock shapes and drew them with the Micron.. In most cases, I looked for areas that were mostly one color, but didn't worry if a rock had more than one color. Many 'edges' were fuzzy. I just ignored those--for example, look above at the turquoise rock in the lower middle. The edges ran profusely. Now look below and you'll see I outlined that rock, leaving the 'fuzzies' on the outside of it. As I added detail, the 'fuzzies' just disappear into the creases and crevices.
Continuing with the Micron pen, I began to add some shadowing, darkening rocks that I felt should be partially under other rocks, and adding some depth to each rock. I tried to make it so some rocks seemed smooth, some rough, some rounded and others jagged.
Next I used two Gellyroll pens - a Classic Purple, and a Metallic Blue to add shadow. Many rocks are shiny and take on slightly different colors as you shift them in the light. I felt the Gellyroll colors would emulate that. In the photo below, you can see how the left half seems more defined than the right. That's the effect of the light reflecting on the Gellyroll colors.
To add a little more definition overall, I went back with the Micron pen and darkened some areas, deepening the crevices.
The last step was to add highlights. My white Gellyroll pen is out of ink, so I decided to use an acrylic interference paint that would change color and character with different light. As with the shadow, I didn't worry too much about where I placed the highlights. I felt that with a pile of stones like this, the rocks would be reflecting off each other and there wasn't much need to worry about the light source.
And that brings us back to the finished picture.