This was way more fun (and easy, all things considered) than I thought it would be.

I filtered the dirt through a section of ordinary window screen -- you can buy a big roll of it at Home Depot for cheap and you'll be amazed how many uses you end up finding for it. The result had a lot more coarse sand in it than I would have thought, and I was worried for a while, but once I'd done the first round of drying and rubbing, all the big sand particles that had dotted the surface of the initial ball of mud just fell right off and the finer clay-y particles (which are stickier) filled in the gaps they left. You can't see them in the pictures, but there are a few really fine particles of shiny sand (quartz? mica?) left in the surface. It looks cool.

Following the directive to make it special, I used an avocado pit (left over from brunch earlier that day) as the core, making this no ordinary dorodango but an unstable seed bomb. I will be observing it over the weeks to come to see if it explodes.