I was actually wondering what made it work. Now that you've mentioned it, if the reddish stone was the same colour as the rest, the composition wouldn't have worked as well. There's also a bit of 'blue-ish' water reflection right below the red stone that's creating something good.
Funny how the little things all add up to hold one's interest. I can stare at that image for quite a while and follow shapes and colours where my eye leads me to. Maybe it's just me... I'm wired like that.