I learned these major lessons from the tutorial: 1) Start at the edges, work inward. 2) Don't be afraid to use the darkest color to "putty in" some awkward corners. 3) Use the next-darkest color all around the rock, not just on one edge for shadow, but use it most on the shadowed side--and don't be afraid to glob it on there. 4) Use that same color for some extra antialiasing around the cracks. 5) Highlights should be pretty irregular--they're rocks after all! 6) Avoid accidentally creating a strong horizontal or vertical line; it's best if you do this part first.
My first attempt was less than successful, but this time I was able to knock out a good stone tile in very little time. A little practice doesn't hurt. This is what I came up with for a first attempt:
That came out pretty well, and with only 4 colors. So I thought to make it look more realistic, less pixel-arty, I would process it a bit. Here's the same image after I ran it through a soften filter, then sharpened twice:
One of the first BYOND games I ever saw did something like this, but with orange rocks that had a fiery tone, and they varied in brightness. So, I worked up a little something along those same lines. I redid my palette so that the crack color was black, and the rocks were shades going from red to orange. Then I added a brighter orange, and a darker red, for a 5-color range plus black. Any given rock used 3 of the colors. I had to be careful not to group the same colors too much, or accidentally create a direction line for the eye to follow.
That came out amazingly similar to what I was trying to emulate, only the rocks are bigger. It looks even better with the same smoothing applied:
Once I'd done this, I realized the same thing could be done to the original gray tones, so I did some quick color swaps on the orange one:
Then I smoothed again:
All in all, I got a nice base to work from if I ever want to use this tile--and variations on it--in a game. It just goes to show that when it comes to pixel art, even if you fail once or twice, it pays to be persistent. I learned a lot from that tutorial, and there is a wealth of others out there.