Do you know the history of how that particular tree was planted on that rock? If it's a Lenz tree, I'd bet the tree was planted over the rock a decade or more ago and left to grow in a field. That's really the only way roots will cling that tightly to a rock (And from what I can see, it's a REAL rock--which makes also pretty big difference, as artifical rocks age like--well--artificial rocks, fading, chipping, scratching, breakdown of material, not to metion pressure from roots gripping the stuff over time. Real rocks age like real rocks
). If timeline isn't the case, then it boils down to being an artist with abilities like Mr. Lenz...which is another post
.
Sticking an artificial rock in a gap between roots and leaving it as a "finished" composition will always look contrived and artificial and will look even more so as time passes.