At my previous workplace, we noticed that certain types of plants would arrive potted in a medium containing mostly lava rock, and these plants would always remain healthier far longer than their counterparts potted in standard soil. This is another medium that provides additional weight, helping to hold heavy and tall plants in place. We source our lava rock locally, from a specific mine. Use caution - some mines produce lava rock that contains too much nickel for use with plants.
I use these lava rock smaller pieces to chunk up my soils, they help with drainage and air circulation to the roots of my plants. I have found these to be a good size for my soil mixture.