This strikes a very similar note to my famous "how to empty and old trough" post from way back in 2012.
On this occasion I can't credit one of my staff with discovering this labour-saving technique, though I should add that the method is broadly similar to my technician's in 2012. But I did this all on my own.
The polished black pebbles we use on almost all our planters are reassuringly expensive, retailing at about £1 a kilo.
If - on returning from an installation with some left over - you have an excess that needs disposing of, for example by evenly distributing over the ground, grab the bag by one end, pick up somewhat quickly and simply allow the other end to burst open.
The crucial technique is to be turning away from the van as this happens for maximum spread.
There, that was easy, wasn't it?