Watching artist Michael Grab balance rocks is really kind of stressful. He moves with excruciating care, holding a teetering pile of rocks steady with one hand while he adds yet another and then another, each at a stranger angle than the last. I had to fast forward through this video the first time, just to deal with the tension of waiting for the rocks to fall:

Here’s how he does it:

notice the technique of bracing the lower rocks while placing the next. the top rock acts as a pin to stabilize the whole structure, which is why it is very helpful to use a LARGE rock on top. plus, i like the overall design balance that the large top rock adds. and i love shades of red.
As soon as i found that red rock, i knew it must go on top. it was a beautiful rock. :) contemplate, try it for yourself. all about presence in the moment. SLOW down.

The results are mind-bogglingly beautiful:

rocks1
Gravity Glue
rocks2
Gravity Glue

More here and here.