the one downside to the rock balancing is how long they last. although to be able to get one of those up and standing for a while is impressive. if you look to the right of one of the pages there are tags that link to many other balances (there are even "gardens" of rock balances) with some videos of them being built. I got lost in the website, just incredible pieces of art and wonderful pictures and commentary.
i do not believe glue is the answer for keeping these up as some of the weights of the top pieces still would not allow it. as mentioned before, small metal rods, unseen, would be a better alternative.