Yeah, I thought you were kidding when you said “extruded shale”. Then I poked around on the internets and that stuff popped up. My mom actually has some in the garage that she uses for drainage in her flower pots. It’s nice and round but is too light for sling-shot ammo. I tried using it to chase squirrels off the bird feeder but it just goes all over the place…
Seeing it used as a hydroponic planting medium made me wonder if we could design green-roofs as an ebb & flow system. Super light planting medium, nutrients in the irrigation, and use the roof as a catch basin to recycle the nutrient solution for the ebb & flow cycle… hhhmmm….