Leaving aside any jokes about testicular fortitude (that I'm not awake enough to think of on my own quite yet), this guy would LOVE my neck of the proverbial woods. A lot of those stones look like naturally-broken shale or slate, probably shale which is less hard, less dense, and more likely to break into small pieces. That's why slate is the one used for chalkboards--it does a much better job of being processed into large thin sheets without shattering. The shale around here--and there's LOTS--comes in a wild variety of colors. Just about an hour away from my apartment there's a huge outcrop right upside Interstate 90 that is partly purple, partly green, a little tan here and there, all in thin layers, and sometimes "intrusive" layers of things like chlorite (bright sparkly non-crystalline green) and calcite (mostly either white or salmon-colored). And there's a pullout right next to it, big enough for a few semi trucks
. Even if Devin does shape the stones by hand, which looks possible from the pics, shale's easy to handle that way too.
I'd never want a place with a "yard makeover". Ugh. Gimmee a stone retaining wall 40 feet long and 15 feet tall at one end so I can slap together the right ceramics and turn it into a mosaic of a Sarcosuchus with the words "Do Not Enter Unless The Crocodile Knows You" between its teeth. If you've never met Sarcosuchus...here ya go:
http://images-cdn.9gag.com/photo/aQ8Mjrd_700b.jpg and with the skull of a modern croc:
http://tinyurl.com/zs4aysa