Lava. Most of the yard is underlain by a lava flow (probably just one). This outcropping was near the surface, and a few years ago I exposed it with a shovel and a pressure washer. Then dust and dirt and cinders partly covered it again, so I got to work with the washer a couple of days ago and cleaned it up.
There's not much to say about this, except that the color variation has to do with what was exposed and what was reburied, both for hundreds or thousands of years, and in the recent past.
I had a hard time finding anything this morning. As I promised yesterday, I went out with the superwide before doing any scouting, and nothing I found really suited my mood. Eventually I wandered over to some of the places where I'd exposed the lava, and this area was the most promising.
One of these days I'll make some establishing shots. If I were a complete nerd I'd grab a Google Earth view and mark where in the yard I made each shot. But I'm not, and I won't.