In the final scene we see Job character revealing the diamonds are fake - glass in fact. An actual diamond would break under a hammer on an anvil just as easily and spectacularly as a fake one. Diamonds are hard but not very resilient to a concussive shock.
When Lucas stops Quentin on the highway, it is a two lane highway with no shoulder. When it cuts to Quentin sitting on his car, it switches to a two lane highway with a passing lane.