This video demonstrates shock diamonds in a healthy, stable rocket engine. As supersonic gasses leave the engine, the gasses encounter the ambient air pressure, which results in a complex interaction where the gasses compress and then expand repeatedly, resulting in the diamond pattern.
On a LOX-Kerosene Rocket Engine, the afterburning in the plume makes it too bright to be able to see the diamonds, but if you iris the camera down very very low, you can see them, as we have done in the camera at the top. A LOX-Kero plume burns about as brightly as the sun as viewed from earth, at sea level atmosphere.
*** MINOR CORRECTION: Video slate states it is a 6 scond run - it is actually an 8 second run.