Forgive my lack of the right terms;
They stapled up some wire mesh, mixed up some commercial grade quickcrete, smeared the concrete on the back of the rocks and smashed them into place. It was all very simple. They were very particular on which stone went where. They did a lot of cutting and chipping to get the rocks just right.
They did suggest that I wait a couple weeks then wash the rock faces off with water, let it dry, then wash with water and a small amount of muriatic acid mixed in. Then wash again with plain water. Said it would really bring out all the color in the rocks.