When I added both of these to my mod as an april fools joke, I made DIRT a powder, and MUD a liquid. But I added some interesting movement code into both of them, so that DIRT doesn't settle like a normal powder pile, it spreads out a bit more, and MUD has some CLST code so sticks together a lot when falling. No idea why I put the effort into a joke element for one day, but it was fun :P