foam is just the material... the look comes from the detail sculpted into it... and finish/ color
how big a rock are you looking to have... and are you looking for it to cover up something you'll place the rock over ... or just looking to have it sit there and look natural... the most natural ones will look like what the native stone in your area is... with it's same coloration..
OH... yes... I've made foam boulders... I did a set for a dance company... that needed a few large boulders that they would dance on in the center of the stage... then they would gather it together onto the plane/truck.. to the next city where they would set it up again for the next show... they needed minimal side strength and great top strength .. and only the hint of rockness.. as with the lighting it was about shadows and configuration.. center stage in the front yard... in the bright daylight.. color and texture will be your greateest help in it looking real..
