A pick-and-place attachment that could locate and remove rocks would be useful. Even better if it scans and stores their shape for later assembly into rock walls, etc.
It could have a little mallet/hammer that taps the rocks to verify that they are rocks (I’m assuming candidate rocks would be detected by computer vision) and maybe get a sense of their size from the vibrations?
I’ve been thinking a lot about this, as it’s something I’m going to need pretty soon. There’s an actuator design that’s basically sand in a balloon, you press the balloon… ah, never mind. I found it, it’s been commercialized: “VERSABALL® Grippers utilize Empire’s proprietary granular jamming technology” ~ Empire Robotics, Inc. So that’s out?
This is what I’m working with. I have no problem building soil on top of of this, but it’s clearly not something one can plow, eh? I won’t be doing a lot of digging on this land. But a robot that could collect, sort, and stack rocks would sure come in handy.
I had a brainwave last night. I think the thing you want here is a shop vac, eh? Suction, not digging, is the way to get at this ground. Dump the stuff onto a set of graduated sieves to get rocks, course and fine gravel, sand, and loam.
A trommel screen is what I want, vacuum the ground into a trommel screen and I’m good-to-go.
Out of interest did you ever get anywhere with this? It looks like a really interesting problem and I’d love to know if your solution worked.
I was thinking another solution could just be something that scrapes the top surface of the soil away, though obviously that would remove the plants and other things on the surface that arn’t rocks.
I haven’t gotten down to brass tacks yet, no. It turned out to be really hot (above 100F) much of the summer so we had to retreat back to San Francisco. I left the motors and other gear up on the land and I’ve been working on other things for the last couple of months.
At some point soon we’ll get back up to the land and I should be able to start trying things out.
Cheers!
Ah thats fair enough, keep us updated on how it goes if you end up getting back to it 
Your stone pathways are about half finished!
Late reply but I love this concept! I recall learning about setting up new farms and one of the big tasks they mentioned was rock clearing.
I will post a project status update in the next month or two, but long story short our vehicle system works pretty well and I will be working on design clean up and documentation, moving Acorn to a 1.0 release. That would be a design other people could make and start hacking on! I have some fun ideas for a heavy duty gantry tool that would be great for this. 
