Making a fake rock isn't uncommon in theatre, and I have done a post previously on options, but of course, as time and projects have past, I have gathered additional options.
This time combing through my files I came across a variety of options:
Superior Rock Solutions which offer panels and classes (on how to use their panels)
Fauxpanels.com offer panel as well, in brick, stone, wood, and other textures (like concrete and asphalt). They also have corners. While the panels seem to line up well, they don't go around corners and the columns are pretty obvious.
Antico Elements has a variety of architectural products including stone, rocks, and bricks (stones being smooth, and rocks being rough)and square column wraps. Again, I don't think these would take a corner well (but this is true for vacuformed panels and fiberglass panels as well (like from Warner Brothers). The pulp art panels seem to do corners the best, as they were fairly solid.
Back in the day, we use to cut grooves into blue foam and melt the edges (ah the fumes), but it worked well. For rocks I have seen a variety of built up options - from homosote to chicken wire and crumpled newsprint. We have also tried molds, but it was a fairly expensive process. Recently we have used a cut stencil (representing the mortar lines) & used that to spray texture onto flats (using dryvit for example, and that created a nice texture fairly easily within the shop.