PopSci has a newer article on this, which is a bit of an update. Some folks are going to use sandstone.
That’s GOOD. Stone is a crappy insulator, which means it picks up lots of heat from the outside, but also keeps really cool in the shade. Seriously, take a stone (not brick) lawn tile, and move it back and forth from sun to shade.
One or two shade trees, and you can blow off the A/C bill.