I once made a BZP topic with the only sound explanation I could drum up, but in hindsight, it’s not really very sound when you realize how contrived it is. You shouldn’t need to bring in even introductory geology to explain your magic system unless it’s explicitly based on geology. I tried writing fanfiction where I applied this rule, and I had to specify in the middle of an action scene that the rocks in a given spot were sandstone, so Onua couldn’t move them. It was super awkward to have to just bring up geology in the middle of a scene that shoulda been about fighting Bohrok.
Try as I might, I fimrly believe that you literally cannot come up with a sound distinction between the two elements. Greg’s defined it as the difference between getting hit in the face with a rock and a lump of dirt; but Onu-Koro is situated in a cave system, which is made of rock…which Onua brings down on the Rahkshi, and which he tunnels through all the time. It’s like the difference between the two elements is WHERE the material is, not WHAT it is–a rule literally none of the others follow. I’m all for giving Stone the boot for G3 and replacing it with, idk, Kinesis so Pohatu can still kick rocks around. The whole elemental roster needs reworking anyway.